Refine document generation

[redboltz]

I've read this thread before:
https://lists.boost.org/Archives/boost//2024/01/255621.php

However, I didn't have much time to study Antora at that time because refining code was my priority. So, I used a tool that was easier for me, Asciidoxy. I expected that it would integrate well with Doxygen, but Asciidoxy is not well maintained. Recent or complex Doxygen-generated XML causes Asciidoxy to error out.

Therefore, I generate HTML using Doxygen and add links to the HTML files from the AsciiDocs. That is the current status.

Asciidoxy is used only for creating multiple HTML files from multiple ADocs. For this purpose, Antora would be better. At least, Antora is well maintained.

Now that the code is refined, I will migrate from Asciidoxy to Antora.