Spec clean up

[cholmes]

Related Issue(s): #1012 #973 #822

Proposed Changes:

1. Consistency in how we refer to Item, Collection and Catalog
2. table of contents added
3. just link to spec constructs once, at the beginning. Link to the actual spec, not the readme.

PR Checklist:

• [x] This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• [x] This PR has no breaking changes.
• [x] I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• [x] This PR affects the STAC API spec, and I have opened issue https://github.com/radiantearth/stac-api-spec/issues/105 to track the change.

[cholmes]

Ok, this is ready for review. I'm sure it could use another set of eyes on similar things. And I didn't go through everything for language - was mostly just updating capitalization and links, but tweaked things where I saw opportunities. But I'd say after review of these changes we should just merge this in and then someone else can take another pass in a new PR.