SEO and social network meta tags

[nweldev]

Add minimum SEO and social network configuration and optimisation.

Partially depends on #7

Minimal Requirements

Not exhaustive at all, but it's a minimum we can start with. See the docs for more details.

• any page can have a coherent and attractive associated card when shared on Twitter (see Twitter Card Validator)
• most shared pages (starting with the home page) have predefined metadata
• we have an associated account for Google Search Console and the result looks OK

How to proceed

See https://docusaurus.io/docs/seo

[nweldev]

Sharing blog posts (where meta tags are well configured) was enough for Prep Q3.
Moving this to Q3 as I'll address it progressively while promoting other parts of the website.

[nweldev]

This is actually more complex than expected. The conclusion is planned for early Q4, as content has been a way higher priority, but this content still needs to be shared on social media pretty soon.