various pages: made absolute links relative

hopefully didn't break anything, some links point to stuff not included
in this repository, hence cannot be tested locally...

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>
fix link to doxygen with link to github

isn't linking to the code way more gooder than linking to some generated
not-so-good html documentation?

Replaced in "device_drivers/writing_driver" non-working link into the
doxygen "api" directory with correct link to correct line of correct
file on github.

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>
Merge pull request #10 from marvin2k/links_relative

Links relative