git.mk: ignore files and folders generated by hotdoc.

This ignores:

+ The hotdoc depdir folder
+ The stamp files generated by the hotdoc fragment
+ The private folder for each hotdoc project
+ The output directory for each hotdoc project

Signed-off-by: Mathieu Duponchelle <mathieu.duponchelle@opencreed.com>

Differential Revision: https://phabricator.apertis.org/D3873