Fix call enum's metadata regression (#3513)

This fixes an issue introduced in
https://github.com/paritytech/substrate/pull/14101, in which I removed
the `Call` enum's documentation and replaced it with a link to the
`Pallet` struct, but this also removed any docs related to call from the
metadata.

I tried to add a regression test for this, but it seems to me that this
is not possible, given that using `type-info` we only assert in type-ids
for `Call`, `Event` and `Error`. I removed some doc comments from a test
setup in `frame-support-test` to demonstrate the issue there. @jsdw do
you have any comments on this?

I also fixed a small issue in the custom html/css of `polkadot-sdk-doc`
crate, making sure it does not affect the rust-doc page of all other
crates.

- [x] Investigate a regression test
- [x] prdoc