Add links to Social contract from contribute and about

E.g. in contribute.mdwn we could:

- [[Code of conduct|contribute/working_together/code_of_conduct]]
+ - [[Social contract|contribute/working_together/social_contract]]
- [[Contributors meetings|contribute/meetings]], and minutes from past meetings

But perhaps it would be nice to also point to it from a page that’s
targeted at users, such as [[about]]?

Feature Branch: tails:master

Original created by @u on 12504 (Redmine)