Tighten docs publishing flow

[danpaul000]

Problem

• We are substituting in a release tag into the docs content based on the latest-in-time release on any branch. We should only sub in tags from the latest tag on the beta branch.
• Docs publishing to docs.solana.com occurs when a tag is made on any branch, but this should only update docs.solana.com when a tag is made on beta branch.

Summary of Changes

• Use ci/channel-info.sh to get latest beta tag, rather than querying github API.
• Add a check to .travis.yml not to proceed with docs publishing if we are not on beta or edge channels.

Fixes #11505
Fixes #11387

[danpaul000]

Commented out the "skip if PR" logic in .travis.yml to test logic. Will revert before merge.

[codecov]

Codecov Report

Merging #12572 into master will decrease coverage by 7.6%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12572      +/-   ##
==========================================
- Coverage    82.0%    74.3%    -7.7%     
==========================================
  Files         356       31     -325     
  Lines       83140     1498   -81642     
  Branches        0      199     +199     
==========================================
- Hits        68197     1114   -67083     
+ Misses      14943      335   -14608     
- Partials        0       49      +49     

[mvines]

lgtm

Pull request has been modified.

[t-nelson]

🎉