Add first section of the TTS Consulting Handbook.

[adunkman]

Includes the first section of content for the TTSC handbook

Changes proposed in this pull request:

• Adds support for section-level navigation in page headers, indicated in the main navigation.yml file.
• Adds previously-approved TTS Consulting handbook content (approvals secured by @SelenaJV)

As a note, this is the first of quite a bit of content that will be landing here, hence the new main section navigation approach.

Security considerations

None, content changes.

[SelenaJV]

Yay! Thank you