rework how version information is presented in combined docs

[heckj]

Summary

Previously, the combined docs had a "version" meant that was used in multiple places - if effected the "slug" for the URL, and we included it into the content itself. The "Swift - main" wasn't useful per feedback, so this switches the synthesized collection to be called "Swift Documentation" generally, continues to use main in the slug since that effects the hosting base path, and then extends that with a string from sources.json that gets hacked/injected in a customized version of the synthesized landing page that the script adjusts. (there's currently no DocC capabilities for authoring the combined landing page - it's synthesized without control, so we adjust it after it's been built in this script)

Local notes include for how we're wrangling that additional JSON fiddling. I don't expect this is the final form, but it gives us some room to adjust and present the Swift version in the combined landing page - or we can strip it out altogether.

Validation

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[davelester]

Can we remove "Swift Documentation" entirely?

[heckj]

Unfortunately no - the "whole collection" has a combined name that matches with the synthesized page, and gets presented in the left nav as the "how to get back to the central location" with the merged catalog setup.

Or more specifically, I haven't figured out a way to work around or remove it, and haven't attempted to suppress it explicitly in the left-nav hacks, since it's a navigable item.

[davelester]

@heckj Thanks for the feedback re: potential removal of the text. LGTM as an iterative improvement, I agree that we can revisit its display separately.