The documentation overview sections is supposed to provide a map
for the rest of the reference document. The getting help and upgrading
sections were moved to align with that.
Using the revision line means that the revision number is automatically
set. The revision number is then included by the HTML and PDF backends
in the standard location. In the HTML backend that's alongside the
list of authors. In the PDF backend that's on the title page.
Closes gh-26851
This commit combines index adoc files for single-page HTML and PDF
into one file to remove duplication. It also renames index files to
reflect the separation between source format and generated output
format.
Finishes gh-20829
