Add an "Edit this page" button

[choldgraf]

Context

When using MyST for knowledge repositories or documentation, it is useful to be able to quickly view the source files for the documentation, and to suggest an edit with as little friction as possible. Right now, doing so requires figuring out the github repository and the location of the source file manually.

Proposal

Add an "Edit this page" or "Suggest an edit" button that provides a link to "suggest an edit" in the version control system of choice for the MyST site.

Tasks and updates

No response