Documentation update

[stuitje]

This PR mainly fixes the broken links to other modules (after the casing discussion we had) and adds a few new features to the SPIDER documentation. This partially addresses the issue FormingWorlds/PROTEUS#635.

• Note blocks are updated to match the style in Aragog and PROTEUS
• New link in navigation, 'Community', now includes 'Contact' (copied and slightly adjusted from PROTEUS), 'Code of Conduct' (moved) and 'Developers' (links to proteus-framework.org/people). Other modules are now linked under 'Other PROTEUS modules' and contains the aforementiones fixed links.
• Publications now includes a link to the PROTEUS publications, as in Aragog
• Installation now links to the 'normal' installation as part of a PROTEUS installation in a note
• PROTEUS logo now links to proteus-framework.org, i.e. the general homepage, via a javascript extra.
• SPIDER title links to the documentation homepage of spider, i.e. proteus-framework.org/SPIDER.

Validation

I built the documentation locally using mkdocs serve and checked all links.

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[nichollsh]

Looks good to me. These info boxes look really cool.