prepare initial contribution to asciidoc-lang

- tailor README for asciidoc-lang
- move LICENCE file into docs
- change Antora component name from "asciidoc" to "docs"
- remove placeholder CHANGELOG file at root
- add Antora playbook for building documentation locally

CHANGELOG.adoc

-= AsciiDoc Language Documentation Changelog
-
-See link:docs/CHANGELOG.adoc[CHANGELOG].

README.adoc

-= AsciiDoc Language Documentation
-:url-prototype: https://github.com/git/git-scm.com/commits/master/lib/asciidoc.rb
-:url-org: https://github.com/asciidoctor
-:url-asciidoc-lang: https://projects.eclipse.org/projects/technology.asciidoc
-:url-docs: https://docs.asciidoctor.org/asciidoc/latest
+= AsciiDoc Language
 
-This repository contains the user-facing documentation for the AsciiDoc language as it's implemented in {url-org}/asciidoctor[Asciidoctor].
-You can view the {url-docs}[published version] online.
-This documentation has been submitted as the initial contribution for the {url-asciidoc-lang}[AsciiDoc Language project at Eclipse].
+This project maintains the specification, Technology Compatiblity Kit (TCK), and documentation for the AsciiDoc language and its APIs.
+The AsciiDoc Language Specification describes the syntax, grammar, and APIs for the AsciiDoc Language.
 
-== Publishing
+== Build the Documentation
 
-This repository only contains the documentation source files and assets.
-The documentation is published {url-docs}[online] for reading.
-Although you can view a preview of the files through the GitHub interface, it's not an official rendering and you should not expect all content to display properly.
-To view it properly, you should read it {url-docs}[on the site where it is published].
-
-== Documentation Structure
-
-This repository is a registered content source in the {url-org}/docs.asciidoctor.org/blob/main/antora-playbook.yml[playbook file for docs.asciidoctor.org].
-The source files are organized into a documentation component as described by the [.path]_antora.yml_ file.
-To learn how to build this documentation using the playbook and Antora, see the https://docs.antora.org/antora/2.3/install-and-run-quickstart/[Install and Run Antora Quickstart].
-
-== Copyright and License
-
-Copyright (C) 2012-2020 Sarah White, Dan Allen, and the individual contributors to {url-org}/asciidoctor/graphs/contributors[Asciidoctor] and {url-org}/asciidoctor.org/graphs/contributors[asciidoctor.org].
-Except where otherwise noted, the content of the AsciiDoc documentation and websites is licensed under a https://creativecommons.org/licenses/by/4.0/[Creative Commons Attribution 4.0 International License].
-The use of the {url-org}[Asciidoctor projects] is granted under the terms of each software project's license.
-
-== Authors
-
-*Asciidoctor* is led by https://github.com/mojavelinux[Dan Allen] and https://github.com/graphitefriction[Sarah White] and has received contributions from many individuals in Asciidoctor's awesome community.
-The project was initiated in 2012 by https://github.com/erebor[Ryan Waldron] and based on {url-prototype}[a prototype] written by https://github.com/nickh[Nick Hengeveld].
-
-*AsciiDoc* was started and maintained by Stuart Rackham from https://github.com/asciidoc/asciidoc/blob/master/CHANGELOG.txt[2002 to 2013] and has received contributions from many individuals in the https://github.com/asciidoc/asciidoc/graphs/contributors[AsciiDoc community].
-
-This documentation for the AsciiDoc language was extracted from the tip of the https://github.com/asciidoctor/asciidoctor.org/tree/docs-archive[docs-archive branch] in the https://github.com/asciidoctor/asciidoctor.org repository.
+The source files under [.path]_docs/_ are organized into a documentation component as identified by the [.path]_antora.yml_ file.
+To learn how to build this documentation using the playbook file [.path]_local-antora-playbook.yml_ with Antora, see the https://docs.antora.org/antora/latest/install-and-run-quickstart/[Install and Run Antora Quickstart].
 
 == Trademarks
 

docs/antora.yml

-name: asciidoc
+name: docs
 title: AsciiDoc
 version: latest
 display_version: current

local-antora-playbook.yml

+# This is a configuration file for Antora.
+# You can pass it to Antora to build this content as an HTML site for previewing offline.
+#
+#  $ antora --clean local-antora-playbook.yml
+#
+# A copy of this documentation is also available online at https://docs.asciidoctor.org/asciidoc/latest/
+runtime:
+  cache_dir: ./.cache/antora
+site:
+  title: AsciiDoc
+  start_page: docs::index.adoc
+git:
+  ensure_git_suffix: false
+content:
+  sources:
+  - url: .
+    start_path: docs
+    branches: HEAD
+  # ...or use this content source instead to pull the content directly from the repository
+  #- url: https://gitlab.eclipse.org/eclipse/asciidoc/asciidoc-lang.git
+  #  branches: HEAD
+asciidoc:
+  attributes:
+    page-pagination: ''
+    experimental: ''
+    idprefix: '@'
+    idseparator: '-@'
+    table-caption: false
+    example-caption: false
+    listing-caption: false
+    hide-uri-scheme: '@'
+    url-org: https://github.com/asciidoctor
+ui:
+  bundle:
+    url: https://gitlab.com/antora/antora-ui-default/-/jobs/artifacts/master/raw/build/ui-bundle.zip?job=bundle-stable
+    snapshot: true
+output:
+  dir: ./public