Redirect a selection of old pages to new ones

[MetRonnie]

This partially addresses #168

Redirects the most important pages in the Cylc 7 and Cylc 8 early beta docs to their current counterparts, so when you switch versions from those to latest, you won't face a 404 anymore.

I think it's possible to do the opposite, but would require manually adding the current counterparts as html files that redirect to the old pages, in the Cylc 7 folders on the gh-pages branch

(Tests will fail until #253 merged)

[oliver-sanders]

Looks good.

[oliver-sanders]

Could convert this logic into JS and inject it into the 404 file, slightly ugly but it saves dumping redirect files about the place and makes redirecting files from removed directories a little easier.

https://github.com/cylc/cylc.github.io/blob/master/404.html