Rename 'latest' to 'dev' about st2docs

[LindsayHill]

Probably OK. Just as long as we catch the various places that need updating.

Shouldn't screw up Google ranking for docs, now that we've been using canonical_url for a while, and most Google search results are looking good now.

[blag]

One way to have documentation across multiple versions is the Django documentation.

Browsing to docs.djangoproject.com redirects you to the current latest stable docs (which is 2.1 as of this comment):

https://docs.djangoproject.com -> https://docs.djangoproject.com/en/2.1

Every major release has its own dedicated documentation:

• http://docs.djangoproject.com/en/1.8
• http://docs.djangoproject.com/en/1.9
• http://docs.djangoproject.com/en/2.0
• http://docs.djangoproject.com/en/2.1

The documentation for the version in development is at dev, and it contains a big yellow warning banner at the top:

https://docs.djangoproject.com/en/dev/

And they have nifty language and version pickers that float on every page.

If we adopt a similar structure, the latest documentation can use canonical URLs to point back to the documentation for the current stable version.

[LindsayHill]

We already have most of those pieces, the only difference is that we also have a copy of docs at/, rather than redirecting from there to /x.y

Happy to look at PR, but right now I don't feel like figuring out what changes are needed in the various release machinery + version picker + warning banners that we have.