Add CI

[tovmasharrison]

Description

I have added a CI and the below tests:

• Make sure there is no 404 where we totally fail to serve the homepage
• Make sure that there are no pages with a conflicting path (e.g. a TMS and an API with the same slug)
• Make sure the served paths are only one level deep (e.g. /swahili not /language/swahili).
• Make sure links to pages are not broken (e.g. missing .md, relative path...)
• Make terms are linked to their pages where possible.

Fixes #557 partially, as tests to enforce the style guide still need to be added.

Checklist:

• [x] I have read the contributing guidelines.
• [x] I have followed the style guide.

[tovmasharrison]

A slight issue occurred when checking broken links. I'm working on it.

[tovmasharrison]

@bittlingmayer All done.

However, just let me know if we need to include find_missing_links.py in the CI.