feat: Starlight ( + "Lemline" doc)

[JBBianchi]

⚠️⚠️⚠️ DO NOT MERGE THIS PR !!! ⚠️⚠️⚠️

What this PR does / why we need it:

• Activates "Starlight" for Astro, the documentation theme
• Integrates @geomagilles Lemline's documentation

Special notes for reviewers:
This draft illustrates how to enable Starlight and how Lemline's documentation looks like 'as is'.
The PR should be split in 2:

• Enable Starlight (author myself)
• Add content (author @geomagilles)

Additional information (if needed):

• Internal links don't work. I didn't fix them yet, and maybe we should agree on the structure before fixing them.
• The order of the items of the sidebar is mostly configured in the articles' frontmatter ("metadata on top"). It probably should be in the astro.config.js file instead.