Fix broken links, anchors and images and enforce CI mkdocs build --strict to fail on broken internal links

[jeromefellus-sekoia]

This PR tries to combat broken links, anchors and images by :

• fixing broken internal links that didn't pointed to the right place
• removing some broken links whose target content didn't exist anymore
• converting most of the full URL links to https://docs.sekoia.io/... to absolute internal links so that mkdocs link verification mechanism can spot more broken internal links
• bumping mkdocs to 1.6 so that we can use the new

validation:
  omitted_files: warn
  unrecognized_links: warn
  anchors: warn  # New in MkDocs 1.6
  links:
    absolute_links: relative_to_docs

recommended configuration as per https://www.mkdocs.org/user-guide/configuration/#validation

• bumping python-markdown to latest ^3.7 version to solve various warnings about etree's deprecated features
• incorporating and rewriting the https://github.com/g-provost/lightgallery-markdown markdown extension to be compatible with python-markdown >= 3.3.7 which prevented upgrading python-markdown
• merging the intakes_by_uuid and modules_by_uuid custom plugins into a single integration_by_uuid to conform to mkdocs policy, as only one plugin shall hook on on_page_read_source (Allow multiple hook files mkdocs/mkdocs#3486)
• compile SCSS files via an in-house libsass mkdocs plugin
• enforce mkdocs bulid --strict to hard fail when a broken internal link is met at CI build

README.md has been updated accordingly

[github-actions]

Pull request was merged, preview was removed.