feat: flexible social media footer links

Here are some MRs which were created to test the incoming feature:

1. eclipsefdn/it/websites/orcwg.org!19 (merged) - An astro site which overrides the default social media config
2. eclipsefdn/it/websites/sdv.eclipse.org!142 - A quicksilver site which does not override the default social media config
3. eclipsefdn/it/websites/dataspace.eclipse.org!12 - An astro site which does not override the default social media config

Hypothetical Breaking Change

There is one edge case which could be considered a breaking change.

If a user previously used all the existing EF defaults, but overrode a single url (e.g. they swapped out for a new facebook url for their project), then they would lose the EF defaults unless they manually added them back in.

But mixing project-specific or industry collaboration-specifc social media alongside EF social media is strange to begin with. IMO, this isn't a use case we need to worry about.