Improve data fetching docs

[amirhhashemi]

• I have read the Contribution guide
• This PR references an issue (except for typos, broken links, or other minor problems)

Description

This PR completely overhauls the data fetching documentation in Solid Router and SolidStart.

Summary

Solid Router

• The actions explanation page has been completely rewritten to include topics such as optimistic UI, error handling, revalidation behavior, and streaming.
• A new queries explanation page has been added.
• Reference pages for useSubmission, useSubmissions, action, useAction, query, createAsync, createAsyncStore, and preload have been updated to improve compliance with Diataxis and enhance accuracy and completeness.

SolidStart

• The data loading explanation page has been renamed to "data fetching" for better consistency.
• The data fetching explanation page has been updated.
• A new data mutation explanation page has been added.
• The data fetching how-to page has been updated to remove redundant content in the introduction.

Proposals and trade-offs

Two proposals were initially considered for this PR. However, during the writing process, I decided not to implement them for the following reasons:

• Data fetching and data mutation how-to guide in Solid Router: The updated data fetching and data mutation pages in the Solid Router documentation already provide practical examples for most common tasks. I did not see the value in creating a separate page that merely rephrases these examples in a how-to format. If a compelling use case is suggested, I would be open to creating this page.

• A comprehensive data fetching explanation page for SolidStart: This proposal recommended a data fetching explanation page that would cover Solid and Solid Router data-fetching mechanisms. However, since the Solid Router documentation already contains a comprehensive explanation, I saw no reason to duplicate content. Instead, I encouraged users to refer to the Solid Router documentation, and focused on the unique features of SolidStart, such as server functions and single-flight mutations.

Related issues & labels

• Closes [Content]: Guide containing useSubmission & useSubmissions #1026, [Content]: Clarify useSubmission(s) Documentation to Emphasize Actions Over Form Submissions #1024, [Content]: Remove the useSubmission/useSubmissions section in the action reference #1003, [Content]: Confusing language for useAction #1192, Proper Usage of action and with Method in Solid Router #791, Server Action: Link to benefits from API Reference #981, [Content]: the preload explanation about cache is very vague How much time does it cache the route or is this only caching the route data not the route itself as after render #1173, [Content]: Clarify how query cache works #1204

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[LadyBluenotes]

I think you did a wonderful job! Most of what I've suggested are just structure errors but the overall content is a massive improvement.