Add documentation for Url source plugin

Tagging as novice.

There are docs in the patch at ✨ Allow callback for Url source, and single item Json plugin RTBC that should be extracted and used here -- omit the final section about the callback property, since that is new in that issue.

adwivedi008 → closed merge request !94

@joachim
I have updated the documentation using https://www.drupal.org/files/issues/2020-05-29/3040427-37-migrate_plus_m... →
and https://www.drupal.org/files/issues/2020-05-17/3040427-32-migrate_plus_m... → as reference as you have mentioned in #5

Closed MR of #7 as that was not correct and was opened by mistake apology for that

Please review the changes and inform if any other changes are required

Hi.

It looks like you've taken the whole of the patch from the other issue. We only want the docs, and not all of the docs.

@jochim updated the MR as per your suggestions
Apology for the bulk changes as I get confused about bringing the whole doc changes vs only document part changes

Moving the issue to needs review.

Nearly there!