tidy-up: URLs (cont.) and mdlinkcheck

- add missing ending slashes.
  To avoid duplicates and to use canonical URLs.
- reapply lost updates.
  Follow-up to 2ae983bf4e #19879
- mdlinkcheck: include the `include` directory.
- mdlinkcheck: show unused whitelist items.
- mdlinkcheck: improve debug output.
- mdlinkcheck: delete redundant whitelist items.
- examples/simplessl: lowercase the protocol part.
- BINDINGS: replace one remaining HTTP URL with HTTPS.
  Issue: https://github.com/pycurl/pycurl/issues/892
- BINDINGS: fix a broken link.
- BINDINGS: follow a refresh content redirect.
- KNOWN_BUGS: whitespace.

Closes #19911

docs/tests/TEST-SUITE.md

@@ -214,13 +214,13 @@ SPDX-License-Identifier: curl
   to drown in output. The newly introduced *connection filters* allows one to
   dynamically increase log verbosity for a particular *filter type*. Example:
 
-    CURL_DEBUG=ssl curl -v https://curl.se
+    CURL_DEBUG=ssl curl -v https://curl.se/
 
   makes the `ssl` connection filter log more details. One may do that for
   every filter type and also use a combination of names, separated by `,` or
   space.
 
-    CURL_DEBUG=ssl,http/2 curl -v https://curl.se
+    CURL_DEBUG=ssl,http/2 curl -v https://curl.se/
 
    The order of filter type names is not relevant. Names used here are
    case insensitive. Note that these names are implementation internals and