change all uses of shall to must, and stylize all non-linked conformance classes

[philvarner]

Related Issue(s): #

• Stylize all conformance class name references with italics instead of code #274
• Replaces uses of shall with must #272

Proposed Changes:

1. change all uses of "shall" to "must". semantics do not change.
2. stylize all non-linked conformance class names. (previously a mix of unstylized and use of code style)

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.