Add a documentation page for advanced low level tricks.
Document the `RIOT_MAKEFILES_GLOBAL_PRE` and
`RIOT_MAKEFILES_GLOBAL_POST`.

I could not work around to have `$(RIOTBASE)/Makefile.include` in the
doc as the `$()` part was removed, so I kept with `$RIOTBASE` for now.

It demonstrates:

 * Adding a module with source code
 * Setting a header include directory
 * Adding dependences, which are evaluated before other modules dependencies

If the application compiles, everything is ok.

Process `Makefile.include` for external modules. It is included after the others
so it could overwrite some of the configuration if wanted.

Process `Makefile.dep` for external modules. It is included before the others so
it could be parsed before setting 'default' values to dependencies.

git checkout can only work if you've already changed to RIOT folder.

Name in the documentation did not match the one in Makefile.include.

Gmane is sadly offline since 2016, so let's link to our own (not so
nice) archive for now, so people can still at least crawl through it.

Modules produce an output archive called `${MODULE}.a` if several modules use
the same name, the output is overwritten.

Exception are made for emb6, nordic_softdevice_ble, openthread and semtech-loramac because their documentation contains warning/issues

The detailed documentation of some functions need the brief description
included to make sense.

Our doc builder at doc.riot-os.org was updated and now the documentation
looks weird (see http://doc.riot-os.org/group__core__msg.html#gad1353dec9af776d4caf5f4e00cec112e
e.g.). This fixes that.

Provides a script to generate a doxygen page from the
`release-notes.txt` and includes it to the doxygen build.

Fixes #6953