Move sharding doc to public wiki

[timmc-edx]

The sharded unit tests readme on edx-platform links to https://openedx.atlassian.net/wiki/spaces/AT/pages/3235971586/edx-platfrom+unit+tests+migration+from+Jenkins+to+Github+Actions#Motivation-for-sharding-manually -- a page that no longer exists on the openedx wiki, since the AT space was migrated to 2U's private instance.

A/C:

• Recreate the page in a new space on the openedx wiki
  • Created https://openedx.atlassian.net/wiki/spaces/PLAT/pages/3869376544/edx-platform+unit+tests+migration+from+Jenkins+to+Github+Actions (may not be final home, but can move more later)
• Update any links on GitHub to the new URL
  • docs: Fix link to sharding doc #33278
• Update any links on the 2U wiki
• Delete the 2U wiki page
  • Not deleted or archived, but contents replaced with a pointer.

[timmc-edx]

Alternatively, maybe this should be converted to an ADR.