[BUG] relative links on the index page don't always work

[MattSturgeon]

Description

When accessing the docs' index directly (as /user-guide/install.html), relative links no longer work correctly (i.e. the links to Home Manager Usage and Standalone Usage).

Minimal, Reproducible Example (MRE)

1. Visit https://nix-community.github.io/nixvim
2. Click a relative link
3. Everything works
4. Open ☰ (ToC sidebar)
5. Click "Installation"
6. Repeat step 2 (click relative link)
7. This time the link is broken (points to e.g. /user-guide/modules/hm.html, note the extra /user-guide prefix)

[MattSturgeon]

I guess we could work around this issue by having a separate index page that isn't install.html?

[MattSturgeon]

We may be able to leverage mdbook's special handling of README files?

Links to README.md will be converted to index.html. This is done since some services like GitHub render README files automatically, but web servers typically expect the root file to be called index.html.
https://rust-lang.github.io/mdBook/format/markdown.html#links