feat: add explanation of patches

[MaferMazu]

Description

This PR improves the explanation of patches. It is related to: https://github.com/openedx/wg-devops/issues/29

Checklist

• [x] Add the patches doc
• [ ] Improve the doc related to patches (Ej: Creating a tutor plugin)
• [ ] Update the changelog.d

Note: Keep an eye on https://github.com/overhangio/tutor/pull/834

[regisb]

Hi @MaferMazu! Are you still working on this?

[MaferMazu]

Hello @regisb. I didn't continue working because #834 may change this PR. If you prefer, we can close this and wait for #834, or I can wait and then retake this effort. I am okay with the two options.

[regisb]

Yes, good point. We'll wait on #834.