[Docs] Rework scripts to allow easier setting of current and past versions

[hasezoey]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

Currently when a new major version is out, fixes like #13147 have to be used to make all paths correct on the gh-pages branch, but as a downside this makes it unable to be viewed locally without reverting the fix, this is a tracker issue (note to self) to improve the script to allow a simple setting of a variable.

Also consider slightly rework on how the "latest versions" are fetched so that it still works for selecting newer versions from branches like 6.x.