Add a dedicated page that covers the AsciiDoc table cell style

Original issue: https://github.com/asciidoctor/asciidoc-docs/issues/75

The AsciiDoc table cell style deserves it's own page. Currently it is only documented as a section on the following page:

Published page: https://docs.asciidoctor.org/asciidoc/latest/tables/format-cell-content/
Source page: https://gitlab.eclipse.org/eclipse/asciidoc-lang/asciidoc-lang/-/blob/main/docs/modules/tables/pages/format-cell-content.adoc

Unlike the other table cell styles, it actually changes how the content is parsed. And there are some things to know about it.

• The AsciiDoc table cell must be used if the content contains AsciiDoc block content (paragraph, listing block, etc); it is not needed if the cell only has inline formatting
• An AsciiDoc table cell is an embedded document
• An AsciiDoc table cell inherits attributes from the parent document (though there are some exceptions)
• Line comments are removed from the content before the table cell is parsed (unlike a normal AsciiDoc document)
• The content should begin on a new line (though this is just a recommendation atm)
• References are shared with the parent document
• Counters are shared with the parent document
• Footnotes are processed independently from the parent document

This issue would include moving the content (or reconciling and removing the content) from the source page linked to above, particularly the a operator section as well as adding some important terminology (most notably "AsciiDoc table cell") and context (when do I need it, when do I not need it).