docs: add fast build feature and activate it in CI

Adds feature for skipping include of doxygen headers into sphinx build
when env variable is set.

Builds incomplete docs (no function API documention)
but speeds up building (20min to 1<min).

Add this fast build as the default way of building docs in CI on
non protected branches.

docs/en/contribute/documenting-code.rst

@@ -513,6 +513,16 @@ As well as wildcards::
 
 Note that this is a feature intended to simply testing and debugging during writing of documentation. The HTML output won't be perfect, i.e. it will not build a proper index that lists all the documents, and any references to documents that are not built will result in warnings.
 
+Fast build
+""""""""""
+Another trick to speed up building is to skip including doxygen generated API documention into the Sphinx build process, skipping this drastically reduces build times.
+
+This can be achieved by adding the fast-build argument::
+
+    ./build_docs.py build -f
+
+or by setting the environment variable `DOCS_FAST_BUILD`. Note that the the `-f` argument is a subargument to `build` and thus must be listed after `build`.
+
 Building PDF
 """"""""""""