The `nixos-render-docs` tool outputs XHTML formatted content. In
order to convince browsers like `firefox` to treat the data as XHTML
the extension must be `.xhtml` and not `.html`.
Using the XHTML-formatted content as HTML is mainly an issue with
self-closing `<a />` tags.
This commit extracts the htmlOpenTool from `nmd` into a module
in the home-manager docs directory. This is done to provide feature
parity with the documentation generated using the docbook .xml files.
Austin Horstman