feat: Link the "Help" and "Contact Us" buttons on index webpage

[barbeau]

Summary:

This PR adds hyperlinks to the "Help" and "Contact Us" buttons in the top bar of the index website:

Right now here are the hyperlinks, although these can be changed:

• Help -> https://github.com/MobilityData/gtfs-realtime-validator/blob/master/TROUBLESHOOTING.md
• Contact Us -> https://bit.ly/mobilitydata-slack

This PR also removes the "About" menu item, as that is ticketed separately in #52 with some work towards it in #84.

Closes #50.

Expected behavior:

Currently when clicking "Help" and "Contact Us" nothing happens. This PR adds hyperlinks to both.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with mvn test to make sure you didn't break anything
• Format the title like "feat: {new feature short description}" or "fix: {describe what was fixed}". Title must follow the Conventional Commit Specification (https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[isabelle-dr]

Looks good, thanks for updating these links!