Improve handling of internal subrepo in plugins.py.

To allow per project dependencies pinning and avoid version conflic with
dev/docs/prod dependencies, remove pinned dependencies.

Add Popup and link css style

Due to subfolder `pagoda/tools` in the path of MKDOCS_ROOT, it was the wrong
`tools` string that was removed in the script.

Update `.gitlab-ci.yaml` and `docs/.gitlab-ci.yaml` to run CI on
`gitlab.liris.cnrs.fr`.

Add bunch of files to define the PAGoDA template:

-  Add worflow configuration files (editorconfig, yamllint, pyproject, etc.)
-  Add MIT license
-  Add python dependencies (pinned and not pinned)
-  Add base documentation

`subs()` method print an error if returned dictionary is empty.
This commit fix it by adding a condition to ensure dictionary is not empty
else return None.

Review documentation, mainly update some links and fix grammatical mistakes.

 v1.0.2 (29/04/2021)

-  Update copyright in license content
-  Improve handling of copyright in plugins.py script
-  Remove useless file from tracked tree

 v1.0.2 (29/04/2021)

-  Update copyright in license content
-  Improve handling of copyright in plugins.py script
-  Remove useless file from tracked tree