[meta] Add source plugin documentation to the codebase

Let's add documentation to the migrate source plugins, similar to #2776179: [meta] Add process plugin documentation to the codebase.

In particular some plugins now use a configuration parameter, for example, 'high_water_property, and should be documented.

The source plugins are;

High priority, uses configuration parameter(s).

• SourcePluginBase - #2862670: Add documentation to SourcePluginBase source plugin
• SqlBase - #2862671: Add documentation to SqlBase source plugin
• EmbeddedDataSource - #2862672: Add documentation to EmbeddedDataSource source plugin

Medium priority

• EmptySource - #2862673: Add documentation to EmptySource source plugin

Next Steps

1. Choose a plugin to document
2. Create child issue
3. Create patch in the child issue. Include the 'availably configuration keys' and an example.