Fix GitHub edit link on every page

[remram44]

master branch was renamed to main, update mkdocs.yml to match.

The link in the top-right corner of every page is currently 404, example: https://github.com/cilium/ebpf/edit/master/docs/ebpf/guides/getting-started.md

[remram44]

Contribution feedback: I am required to sign-off commits, but there is no explanation anywhere on what I'm signing:

There's a helpful link to this page that states:

Before signing off on a commit, you should ensure that your commit is in compliance with the rules and licensing governing the repository you're committing to. The repository may use a sign off agreement, such as the Developer Certificate of Origin from the Linux Foundation.

However there is no information about this in your CONTRIBUTING guide. If you expect that people are signing documents you never presented to them, you should know that is not how signing works, and your whole process is probably legally moot.

[lmb]

Thanks for your feedback. You're right that we should mention that we indeed need you to adhere to the developer certificate of origin: https://developercertificate.org/ It's not about signing a document but signalling that you have read and understood the DCO.

[lmb]

Thanks!

[lmb]

See #1396