Add local build system for offline use

[dermotcanniffe]

Adds submodules and a build.py script to serve the site locally with relative links. Since the website is down (giving 403 errors), this system will allow people to access the docs and installers even when the site is down. It loads the install and docs sub-domain repos as sub-modules, and builds the site into a dist/ subdir, swapping the hrefs so they point to local media. It also builds the docs repo with mkdocs.

[ThomasFarstrike]

Interesting!

Indeed, the website was done, the shared hosting was often giving issues so I moved it to a VPS now.

But I'm not sure if this pull request isn't overkill... the website already works by just opening index.html
The docs and install repos also build and work locally, unless I'm missing something.

What doesn't work is opening index.html locally and clicking the links to the docs and install subdomains, indeed.

This pull request fixes that for sure, so it's an improvement, but I'm just doubtful that it's worth the added complexity, not forgetting to update the submodule pointers whenever the docs and install and chat repos change, etc...

So I'm wondering @dermotcanniffe , how much do you care that this gets merged? Would you mind if we don't, especially now that the websites are rock solid with the new hosting?

Thank you for chiming in on this!