Improve documentation for workspaces

[krassowski]

In review of #15946 @JasonWeill also reviewed the pre-existing bits of documentation on workspaces. I will commit the suggestions that were made as code suggestions, but because I would like to keep that PR focused on the workspaces UI rather than rewriting existing docs, I will list the remaining suggestions in this issue to allow for more discussion and pick them up later:

jupyterlab/docs/source/user/urls.rst

Line 253 in a1166c1

Note the part of the data key after the first `:` (`package.json:JSON`) is dropped and is irrelevant.

This needs to be clarified or rewritten. The term "irrelevant" is very concerning in user documentation; clearly, the part after the first : is relevant to something — why else would it be there?

Originally posted by @JasonWeill in #15946 (comment)

jupyterlab/docs/source/user/urls.rst

Line 220 in a1166c1

Also any plugins that register with the `ILayoutRestorer` will look up all keys in the State DB

This is not clear. We should use "state database" as the shorthand for IStateDB (even though it is longer in characters, it is easier to read). Although this refers to "many plugins", it's not clear what a user would find important about this. The final sentence also trails off; it is not clear what is mapped to what.

Originally posted by @JasonWeill in #15946 (comment)

jupyterlab/docs/source/user/urls.rst

Lines 200 to 204 in a1166c1

	The ``import`` functionality validates the structure of the workspace file and
	validates the ``id`` field in the workspace ``metadata`` to make sure its URL is
	compatible with either the ``workspaces_url`` configuration or the ``page_url``
	configuration to verify that it is a correctly named workspace or it is the
	default workspace.

This is one long sentence. Can we rewrite it to be clearer about what is most important?

Originally posted by @JasonWeill in #15946 (comment)

[krassowski]

For reference, there is not much in git history to explain the missing bits, it was all added in #7945.