docs: update grass-stable URL to grass-devel

[neteler]

Update manual URLS from grass-stable -> grass-devel.

Addresses #5162 (partially)

[echoix]

It is fine, but we still need to keep in mind the method to rewrite them on release branches. It can be solved later on though

[neteler]

It was a ag and sed one-liner, should be easy.

What I did:

ag --markdown "grass.osgeo.org/grass-stable" -l | xargs sed -i 's+osgeo.org/grass-stable+osgeo.org/grass-devel+g'

[wenzeslaus]

Wouldn't be better to have the "stable" URLs everywhere regardless of a version and then change them during the doc build if needed, e.g., by adding that one-liner to the CI doc build? That way, the source code can be the same on all branches.