Skip the compiler workflows for pull requests that change no build input

ubuntu.yml, macos.yml and windows.yml run about 155 jobs on every pull
request, including those that change only the documentation. They now
have a pull_request path filter: it starts with '**' and excludes the
documentation, Markdown files, GitHub metadata, other workflows and the
license files, so a new file runs them unless it is excluded on purpose.
Pushes to develop, master and release/* are not filtered.

The documentation jobs and the REUSE check move from ubuntu.yml to the
new repository_checks.yml, which has no path filter and also runs
tools/check_path_filters. That check keeps the three filters identical
and fails when a file the compiler jobs keep mentions an excluded file.
It found two: tools/check_build_options reads the Meson section of
package_managers.md, and the swiftpm job builds the Swift example from
the documentation; both are included in the filter.

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

.github/CONTRIBUTING.md

@@ -154,6 +154,11 @@ and each page's title (H1) and declaration by
 [`docs/docset/generate_docset.py`](https://github.com/nlohmann/json/blob/develop/docs/docset/generate_docset.py);
 `make build` reports API pages that cannot be classified.
 
+A pull request that changes only the documentation, Markdown files, GitHub metadata, or license files skips the
+compiler workflows (Ubuntu, macOS, Windows); the documentation and license checks still run. See
+[`tools/check_path_filters`](https://github.com/nlohmann/json/tree/develop/tools/check_path_filters) for the list of
+skipped files and what to do when a compiler job starts reading one of them.
+
 ### Amalgamate the source code
 
 The single-header files