Update the documentation manifest to use relative links instead of pointing to "master" explicitely.

[youknowriad]

This will allow building the handbook from several branches.

cc @chrisvanpatten @mkaz @nosolosw

[oandregal]

Core ticket: https://meta.trac.wordpress.org/ticket/4244

[oandregal]

@youknowriad was looking into this today. I saw the core patch to update the branch the contents are pulled from in the plugin.

I'm offering a different approach here that doesn't require adding steps to the WordPress release process:

• PHP docs importer will stay as it is. No changes necessary during WordPress releases.
• Manifest markdown_source entries are updated after the WordPress release to point to the latest stable branch. Example: Pin branch used to build handbook #14699

[oandregal]

PR ready at #14739

[oandregal]

Closing this as the DevHub's handbook uses relative links.