Rename 'latest' to 'dev'

[arm4b]

At https://docs.stackstorm.com/ we show latest link for the current st2docs master which is development branch.

The proposal is to rename it to dev or similar, because it's more explicit and latest might be understood as latest stable.

This will also require a path update for the st2cd machinery.

[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.