feat: updates groups help center link

[katrinan029]

Description

Updates links to the groups help center doc and adds a subtitle in the Create group modal.
JIRA

Testing

1. checkout branch and run npm run start:stage
2. navigate to https://localhost.stage.edx.org:1991/alc-general/admin/people-management and click on Create group. verify that you see the subtitle and clicking on the Learn more link takes you to the help center.
3. Upload a CSV file with a random email in the Create group modal so that an error card will appear. Click on the the link and verify that it navigates you to the help center.
4. Navigate to a groups detail page and click on Add members. Verify that clicking on the Learn more link in that modal takes you to the help center.

• Ensure adequate tests are in place (or reviewed existing tests cover changes)

Only if submitting a visual change

• Ensure to attach screenshots
• Ensure to have UX team confirm screenshots

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.43%. Comparing base (64429eb) to head (c11229b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1416   +/-   ##
=======================================
  Coverage   86.43%   86.43%           
=======================================
  Files         660      660           
  Lines       14888    14889    +1     
  Branches     3162     3162           
=======================================
+ Hits        12869    12870    +1     
  Misses       1947     1947           
  Partials       72       72           

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