fix redoc build and cleanup warnings

[philvarner]

Related Issue(s): #165

Proposed Changes:

1. change redoc build to use openapi-cli instead of swagger-cli, which should fix the issue with the published version of
2. replace index.html with current recommended content from ReDoc
3. cleanup redoc warnings

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.