content: source track v.next draft, address remainder of pre-merge issues

[zachariahcox]

closes out the remainder of the pre-merge issues.

changes

1. adds high-level document status section.
2. add outstanding TODOs from ☝️ gdoc
3. add link to label:source-track issues in slsa repo
4. removes reference to "time" in the definition of "revision."
5. adds source track links to what's new file.

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[TomHennen]

LGTM thanks!

[TomHennen]

It looks like I technically have permission to merge this, but I don't think that's the right process. Assigning to the 'specification maintainers' for their guidance.

[joshuagl]

It looks like I technically have permission to merge this, but I don't think that's the right process.

Thanks for holding off! Per the CONTRIBUTING guide, content changes should have 3 approvers and allow a minimum of 72h for review: https://github.com/slsa-framework/slsa/blob/main/CONTRIBUTING.md#pull-request-types

[joshuagl]

Thanks, this LGTM but please restore the TODO items within the file contents.

[lehors]

LGTM