Apply standard header case convention in docs

[jbogarthyde]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

The Google doc guideline to use sentence-style capitalization for page titles is not in the Angular doc guidelines, and is not applied consistently.

Issue Number: #21605

What is the new behavior?

• Adds the recommendation to the guidelines
• Applies the correct case to all pages
• Fixes some incidental terminology and organizational inconsistencies encountered while touching every page.

Does this PR introduce a breaking change?

• Yes
• No

[jbogarthyde]

This is a docs-only change and does not actually require technical review from every code-owner.
@kara should look at it, since it includes some terminology/doc org corrections for ivy/ngcc.

[kapunahelewong]

Wow, thanks for this Judy. Did you mean to delete ngcc.md? Otherwise LGTM!

[mary-poppins]

You can preview e4a86df at https://pr34196-e4a86df.ngbuilds.io/.

[mary-poppins]

You can preview ea0a5d8 at https://pr34196-ea0a5d8.ngbuilds.io/.

[jbogarthyde]

Did you mean to delete ngcc.md? Otherwise LGTM!

Yes -- it was a whole new page that just had a 2-sentence definition of the term, and sat uneasily in the CLI section. I added the definition to the glossary (along with "Ivy") and replaced the references to the page with references to the glossary definition.

[mary-poppins]

You can preview f232db2 at https://pr34196-f232db2.ngbuilds.io/.

[mary-poppins]

You can preview 03aa42c at https://pr34196-03aa42c.ngbuilds.io/.

[mary-poppins]

You can preview 535054a at https://pr34196-535054a.ngbuilds.io/.

[mary-poppins]

You can preview dd9a6ae at https://pr34196-dd9a6ae.ngbuilds.io/.

[petebacondarwin]

LGTM!