fix favicon & MD linting; add docs/html to .gitignore

[2bndy5]

Saw some easy fixes about the docs, so I figured I'd submit these changes:

• Properly use favicon.ico which reuires some extra info in the Doxyfile

  On a side note, I noticed the logo is using a white background which kinda sticks out in the docs when using the dark theme. I didn't see the logo image's source file, so I didn't make any changes to the logo. I was able to hack something with GIMP, but the letters (especially the "e") looked rather ugly without the white infill.

• Locally, I use the markdownlint extension in VSCode. This repo's README.md had some "violations" (linter's term - not my preferred term) that were easily addressed.
• The generated docs can be inspected locally by running

  doxygen

  at the repo's root folder. This technique also requires that the gitignore file exclude any changes to the Doxygen HTML output folder (specified in the Doxyfile as docs/html).

---

I noticed the Doxyfile was generated (or at least last updated) by Doxygen v1.9.1. However, the CI is running v1.9.3. The Doxyfile should easily be updated by running

doxygen -u

This is not that important, and IDK which version of doxygen you're using locally. This is why I'm only mentioning it but not doing anything about it.

[2bndy5]

FYI can be obtained by clicking the settings icon () and filling in the info You also don't need to display the "packages" data because this lib wouldn't apply to that. The "environments" data can be used to display the deployment history of the gh-pages branch. Although, this repo updates that on every commit, so maybe its not worth displaying that.