doc: enable source links in examples

This is a bit more convoluted than usual, because we want documentation
built for tagged releases to point at the source code for the release,
and master otherwise. We rely on version having a '.dev0' suffix (or
'.<anything>', really) once we've deviated from the latest release.

The first formatting pass also requires us to wrap {source} and {line}
in extra {}'s.

doc/conf.py

@@ -61,6 +61,11 @@
 
 hawkmoth_root = os.path.join(os.path.abspath(os.path.dirname(__file__)), '../test/examples')
 
+source_uri = 'https://github.com/jnikula/hawkmoth/tree/{version}/test/examples/{{source}}#L{{line}}'
+source_version = f'v{version}' if len(version.split('.')) == 3 else 'master'
+
+hawkmoth_source_uri = source_uri.format(version=source_version)
+
 # -- Options for HTML output -------------------------------------------------
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#options-for-html-output