In GitLab, the project description contains a link to the Cassini
documentation which is hosted on readthedocs.io.

A GitLab UI change made this less prominent than the contents of
README.md, which does not link to the Cassini documentation (it mainly
contains instructions on how to build the documentation)

Move the instructions on how to build the documentation from README.md
to the developer guide and replace with a link to the actual Cassini
documentation on Read the Docs.

Changelog: other
Signed-off-by: Adam Johnston <adam.johnston@linaro.org>