docs: add reference documentations

[JohelEGP]

Resolves #395.

This is a WIP for the reference documentations. It's in a state that can build the PDF and HTML. @mpusz Please, use this PR to test #532.

Instructions:

1. Checkout these repositories:
   • https://github.com/JohelEGP/jegp.cmake_modules/.
   • https://github.com/JohelEGP/draft/tree/standardese_sources_base.
   • https://github.com/JohelEGP/cxxdraft-htmlgen/tree/standardese_sources_base.
2. Satisfy the requirements:
   • For the PDF, see the README: https://github.com/cplusplus/draft.
   • For the HTML, see the README: https://github.com/Eelis/cxxdraft-htmlgen.
3. Consider https://github.com/JohelEGP/jegp.cmake_modules/blob/0df2908dfe83b1f95920d2b20d9436b8f8650498/README.md?plain=1#L107-L121.
4. Configure:

   cmake -S docs/reference -B build/docs/reference \
   -DCMAKE_MODULE_PATH="<path to gh:JohelEGP/jegp.cmake_modules>/modules" \
   -DJEGP_STANDARDESE_SOURCES_GIT_REPOSITORY="<path to gh:JohelEGP/draft>" \
   -DJEGP_CXXDRAFT_HTMLGEN_GIT_REPOSITORY="<path to gh:JohelEGP/cxxdraft-htmlgen>"

5. Build as normal. For the HTML, you may need to put mathjax-node-cli in PATH.