Update mdbook

[meator]

closes #840

I don't have much experience with HTML/CSS, feel free to take over this PR.

The first part of this PR "rebases" the theme files from base mdBook v0.4.14 to mdBook v0.4.43 (current version of mdBook package). This was done for several reasons:

• Adding newer and newer things on the old base can have negative effects.
• I split this update into two commits. The first commit overwrites void-docs theme files with the default ones and the second one applied void-docs specific changes. The second commit nicely shows how void-docs configuration differs from the default one (this comparison is much more difficult to make with the current code, but I did it, see "Void's changes to mdBook's default theme" in Building documentation using recent versions of mdBook is slightly broken #840).

The next step is to actually fix the issues in #840. You can see my attempts in my "UNFINISHED" commit.

Things to consider after solving this issue

The ghcr.io/void-linux/infra-mdbook container will have to be updated.

Once that's done, this line will have to be updated to point to the new container. I am not closely familiar with Void's infrastructure, more steps may be necessary.

The void-docs package should be revbumped once this issue gets resolved.