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

prdoc/pr_3513.prdoc

@@ -0,0 +1,18 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Fix call enum's metadata regression
+
+doc:
+  - audience: Runtime Dev
+  - audience: Runtime User
+    description: |
+      This PR fixes an issue where in the metadata of all FRAME-based chains, the documentation for
+      all extrinsic/call/transactions has been replaced by a single line saying "see Pallet::name".
+
+      Many wallets display the metadata of transactions to the user before signing, and this bug
+      might have affected this process.
+
+crates:
+  - name: frame
+  - name: frame-support