Pull md files from doc source repo instead of wiki

[AlanDrake]

Preliminary step to #242:

• Linking to another repo instead of the wiki.
• Added an edit link to github above the footer.

Notes:

• links from github preview don't work because it expects a .md extension
• since the source is still pulled through a submodule, we still have the same pro/con as before that it checks out the same submodule version which was committed

[ericoporto]

links from github preview don't work because it expects a .md extension

I can't understand what this line means...What is GitHub Preview?

[AlanDrake]

What is GitHub Preview?

The preview from the normal file editor. It's almost the same to the wiki editor, but they treat links differently.

[ivan-mogilko]

I have a question, how is the manual-source repo updated currently? is it synced with .wiki automatically, or someone cherry picks the commits?

[ericoporto]

Right now it's done manually and I think the idea would be in the future to remove the wiki and work all in the repository.

This is only part of the problem, there is still the issue of figuring what to do with the online ags-manual. I think the idea is once at least the sources of the manual is using a regular repository at least on that side branches are more useful and then it would be on this side figuring what to do with the different branches.

[AlanDrake]

Yes, I was currently syncing manually until this PR is merged, moving the editing inside the branch md files, so we can also make new branch for ags4.

Once we have two branches, we can see how to deal with multiple versions of the docs and bundling them to the editor releases.