Replace twitter link with mastodon #415

[phillxnet]

Fixes #415

This pull request's proposal

Our new social contact is to be on Mastodon so remove as-yet unused Twitter and replace with newly created Mastodon icon/links/url.

Includes:

• Adding Mastodon verification link-back into head.
• Fontawesome upgrade from 4.3.0 to 6.3.0 to get the Mastodon icon.
• Use of fa-brands css/webfonds only: we would only use the back and forward arrow from the much larger fa-solid icon set.
• Use text approximation of back and forward arrow in navigation footer, see previously item.
• Adapt/simplify style.css to account for new social_icons and provide a hover background effect.
• Minor index/FAQ re-wording to improve consistency of recent license clarifications, and base OS version updates.

Checklist

• [x] With the proposed changes no Sphinx errors or warnings are generated.
• [x] I have added my name to the AUTHORS file, if required (descending alphabetical order).

[phillxnet]

We have a new unrealated link-check failure here: https://opensource.org/licenses/Apache-2.0 Ignoring for now as this may be resolved upstream, or we can address in a follow up pull requests. The following link: https://opensource.org/license/apache2-0/ appears to work. So they have dropped the trailing 's' in the prior 'licenses', and removed the capitalisation in 'Apache' and dropped the first '-'.

However on further examination our other license link has a working re-directs to the new structure. Lets let this settle a little while first to see if they add the new re-direct. We may want to correct all our links to their final url however.

[EDIT]: strike-through in above and follow-up commit to ajust our links to the actual and likely missing redirects.

[phillxnet]

PR product PRODUCTION published.