Docs: Full site editing templates, template parts and custom templates

[carolinan]

Is your feature request related to a problem? Please describe.

The addition of custom templates in #27778 did not include documentation, and this needs to be added.

In the developer documentation, "Theming for the Block Editor" at https://developer.wordpress.org/block-editor/developers/themes/ neither the default templates, template parts or the new custom templates are documented.
These are only documented in the tutorial at https://developer.wordpress.org/block-editor/tutorials/block-based-themes/ ?

[carolinan]

In the glossary https://developer.wordpress.org/block-editor/handbook/glossary/,
Block Templates link to https://developer.wordpress.org/block-editor/developers/block-api/block-templates/
which is not the same as a template in full site editing.

[carolinan]

This would be partially solved with #25839

[carolinan]

Documentation for custom templates was added in #28830