More documentation updates on the Ibex to CVE2 conversion

These are some remaining necessary updates to the documentation of the CVE2 regarding the undue references to the Ibex project/core, part of #54 (closed). I want to have this PR reviewed before submitting a PR regarding the changes to the code documents (which will include RTL code).

In summary:

• More substitution of Ibex to CVE2 occurrences when it is due.
• Substitution of OpenHW Group for OpenHW Foundation when it is due.
• Reference to the CORE-V-VERIF port of the CVE2, the cv32e20-dv repo, as the default functional verification environment.
• (More) references to eXtension InterFace, as an optional feature.
• Update the Sphinx Makefile variable for the project from Ibex to CVE2 - This single change may crash the Read the Docs, so another reason to have this PR reviewed and applied before next changes.
• Fixed some typos