Format-check the documentation examples in CI

The examples live in docs/mkdocs/docs/examples, but both format checks
still referenced the long-gone docs/examples path:

- check_amalgamation.yml passed it to find, which printed an error for
  the missing path and carried on, so astyle only ever saw include and
  tests. The step still exited 0.
- ci.cmake globbed it into INDENT_FILES, and a GLOB_RECURSE over a
  missing directory silently yields nothing, so the ci_test_amalgamation
  target skipped the examples too.

Either way the 231 example files have never been format-checked. Point
both at the real path, and guard the workflow with an explicit directory
check so a future rename fails the job instead of quietly shrinking the
file list again.

Also drop the dead docs/examples/** path filter from
publish_documentation.yml; docs/mkdocs/** already covers the examples.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/publish_documentation.yml

@@ -7,7 +7,6 @@ on:
       - develop
     paths:
       - docs/mkdocs/**
-      - docs/examples/**
   workflow_dispatch:
 
 # we don't want to have concurrent jobs, and we don't want to cancel running jobs to avoid broken publications