Library detail page: render README.adoc , v2

Issue 238 wrote:

"""
Individual Library detail pages should show the Asciidoc file README.adoc (if it exists) rendered into HTML matching the style of the site-docs and library-docs. This would come last, after all the statically generated stuff that is coming from the database.
"""

What is the expected result if a boost library has a README.adoc, and no README.md ?

Logically... the library details page ought to render and display README.adoc, rather than render and display nothing.

On boost 1.91.1 beta 1 boostorg/url is broken, since it has a README.adoc, and no README.md.
No README is shown.

The issue identifies boostorg/url's README.adoc and the library detail page as the reproduction case; start by reproducing that page when no README.md exists. Trace how README files are selected and rendered, and consider the work done when README.adoc appears in the same place and style as other library documentation.

Written by the indexing model from the issue text.