docs: fix edit url to use main branch

[shuuji3]

Hello 🙂, this PR fixes a broken URL of the edit button on the documentation.

If you click the edit button on the documentation page, you will be redirected to a URL like this: https://github.com/argoproj-labs/argocd-autopilot/edit/master/docs/index.md. But we could not access the page since the branch name was renamed from master to main at some time.

To fix the issue, we need to explicitly specify the edit_uri option of the mkdocs, which uses the master branch by default.

I confirmed this change works on my local build by running make serve-docs command:

[CLAassistant]

All committers have signed the CLA.

[roi-codefresh]

Thank you @shuuji3!

[shuuji3]

Thanks for merging and developing the autopilot! I'm currently enjoying my first argocd-autopilot deployment. 🙂