feat: added help section for post documentation

[sundasnoreen12]

INF-821

Description

To improve learner access to documentation about mathjax and forums in general, we introduced links to these docs within the text editor as show in option #7 of this mockup: Discussions and shown below.

Clicking on the “?” icon at the bottom right of the text editor will open a help menu. Users should be able to close it, similar to preview.

Links to 2 docs will be shown initially, but more may be added later.

The “Participating in course discussions” will point to this link: Participating in Course Discussions – edX Help Center

The “Entering math expressions in course discussions” will point to this link: Entering math expressions in course discussions

Screenshots/sandbox (optional):

Merge Checklist

• [ ] If your update includes visual changes, have they been reviewed by a designer? Send them a link to the Sandbox, if applicable.
• [ ] Is there adequate test coverage for your changes?

Post-merge Checklist

• [ ] Deploy the changes to prod after verifying on stage or ask @openedx/edx-infinity to do it.
• [ ] 🎉 🙌 Celebrate! Thanks for your contribution.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.18%. Comparing base (4297a96) to head (ab1b07e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #733 +/- ## ========================================== + Coverage 93.16% 93.18% +0.01% ========================================== Files 160 161 +1 Lines 3396 3404 +8 Branches 921 919 -2 ========================================== + Hits 3164 3172 +8 Misses 214 214 Partials 18 18 ```

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