doc: introduce doc_layout

Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>
nitdoc: use doc_layout for header html generation

Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>
Merge pull request #275 from itch76/wip-nitdoc

Wip nitdoc
doc_layout: added DocFooter class

Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>
nitdoc: use doc_layout for footer html generation

Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>
Merge branch 'wip-nitdoc' of https://github.com/itch76/nit into wip-nitdoc