Make UAST v2 the default specification in documentation links.

[creachadair]

This is an attempt to clarify the status of UAST v2 in response to #236. This is intentionally not a very large change—there is a lot of other cleanup we could do here but I have not tried to fold it together too tightly.

For now:

• Rename the UAST v1 and v2 spec files to a common format.
• Update the summary to link to v2 as the default,
• Update v1 to more strongly indicate that v1 is being replaced and that v2 should be referred, and
• Update existing spec links to point to the v2 base.

Suggestions are welcome.

[creachadair]

I'm going to go ahead and merge this; @juanjux if you have any concerns I'm happy to send a follow-up.