chore(docs): add useFragment_experimental documentation

[alessbell]

Closes #10056. Adds useFragment_experimental documentation.

This PR:

• adds a Hooks (experimental) page to the docs which documents AC web's first experimental hook, useFragment_experimental
• adds a section to the bottom of the fragments page
• mention useFragment_experimental in the table at the top of the cache interaction page and section that points people to the API reference for the hook

To do:

• update types for options/result after discussing with @benjamn - PR chore: omit previousResult, lastCompleteResult from options/result #10100
• add section on useFragment_experimental to cache-interaction page

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[StephenBarlow]

Approved on the docs side with suggestions!

[jpvajda]

@StephenBarlow thanks for all the great bits of feedback. 🙏 I like this a lot, but I'm curious if we could add a clear use case or example as I find it nice when we say things like:

Let's say you want to do X with Y... Well here is how you can with this new thing we are releasing...

@alessbell

[StephenBarlow]

@jpvajda That addition might make the most sense as a separate section (marked "experimental") in this doc: https://www.apollographql.com/docs/react/caching/cache-interaction

The hook is always for reading and reacting to changes in cached data, correct?

[alessbell]

Thanks for the review, @StephenBarlow! I like the idea of adding a section to the cache-interaction page - I should be able to wrap this up tomorrow 😄

[benjamn]

Details 😈

[benjamn]

Suggested change

	**Required.** The cached object (or object reference) for the fragment being retrieved.
	**Required.** An object containing a `__typename` and primary key fields (such as `id`) identifying the entity object from which the fragment will be retrieved, or a `{ __ref: "..." }` reference, or a `string` ID (uncommon)

Need to be a little more specific than "the cached object"

[alessbell]

Thanks, +1 for the details and updated the PR 👍 I took this from the InMemoryCache docs section on the identify method's signature and would be happy to update it there too.

[benjamn]

Suggested change

	An object or string containing an error message or path to cached items with corresponding error messages.
	A tree of all `MissingFieldError` messages reported during fragment reading, where the branches of the tree indicate the paths of the errors within the query result

I should mention I'm especially open to feedback on this part of the (experimental) API. The MissingTree can be a lot more compact than a list of error objects, but some details require explanation, like how arrays are represented as objects in the MissingTree (because they tend to be sparse).

[alessbell]

Makes sense, made that update - I'm curious about the best way to solicit feedback here but that's a more general question about the way we gather input on features in beta/preview.