Clean up topics on DITA 1.2 and DITA 1.3

[robander]

The content in these topics is quite outdated, and gives no-longer-useful information such as what version of DITA-OT 1.x added specific features of DITA 1.2.

This pull request cleans up the two topics, adds a bit of new information, updates links from the base spec to errata 01, and moves all of the links to keys.

The TOC currently has DITA 1.2 before DITA 1.3, which feels like it's "in order" but I'd suggest flipping that order because 1.3 support is far more important / relevant than 1.2 support at this point. Haven't put that in this pull request though because I know those maps are heavily in flux.

Another long-term cleanup item -- I noticed some of the existing keys for the spec use names like dita-ot-spec but should really be dita-spec or dita13-spec (because it's the DITA spec, not the DITA-OT spec). But that's pretty trivial and I don't want to cause churn by changing at this point.

[infotexture]

🙇 Thanks, very welcome cleanup indeed.

Good call on deferring changes to ToC order. Once #162 lands, we can implement this change (if it's not already included there).

As for changes to spec key names, I'll check how widely they're used outside of these topics to assess the effort/risk of changing them. I have a hunch it'll be worth doing now, but we can treat it as a separate task.

We don't really have a taxonomy in place for key names at this point. They seem to have grown over time with a variety of syntaxes and conventions. Worth harmonizing at some point, but later…

[infotexture]

Much better, thanks.

I checked and found the dita-ot-spec* keys are only used in a few other places, so I'll fix those now.