Add optimistic extension to extensions index

[lorenseanstewart]

Description

Added hx-optimistic to the community extensions list.

Corresponding issue:

Testing

none

Checklist

• [ x] I have read the contribution guidelines
• [x ] I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• [ x] This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• [x ] I ran the test suite locally (npm run test) and verified that it succeeded