Update doc

rename all urls pointing to old gitlab.irstea.fr:

• doc
• readme
• ci

also:

• mention the big otbtf/keras tutorial somewhere

added Documentation label

changed title from Migrate to gitlab inrae to Update doc

marked the checklist item doc as completed

marked the checklist item readme as completed

marked the checklist item ci as completed

marked the checklist item mention the big otbtf/keras tutorial somewhere as completed

mentioned in merge request !116 (merged)

mentioned in commit 595f9d41

closed with merge request !116 (merged)