Restructure Upgrading

[zerocrates]

The Upgrading instructions have several caveats throughout that aren't really relevant for the vast majority of users.

We should probably just pull all those notes out into a separate heading, to say OK, if you're using 0.9 (!) then db.ini is here, if before 2.0 "files" is "archive", etc, without cluttering the basic instructions.

[allanaaa]

Sounds good. Also an argument for versioning the docs.

[zerocrates]

The Upgrading page is a bit of an odd duck in that it's always going to have some interest in talking about older versions, even in a hypothetical versioned-docs world.

There's also the reality that the versions that have relevant-for-upgrading differences are all really old: we haven't changed anything in this area for a long time and aren't really likely to do so.