Add documentation for Table source plugin

Problem/Motivation

The task is to add a documentation for Table source plugin, provided by Migrate Plus module.
Please follow the meta Drupal core issue #3189481: [Meta] Add source plugin documentation to the codebase and its child issues for more details and examples.

Proposed resolution

Add documentation following the same pattern as Drupal core plugins.

Remaining tasks

Publish a merge request, review and commit.

The task is created for our local event - Lutsk Code Sprint, please ignore if you're not a participant. Thanks!