[FWE] Update site structure and sidenav

[ericwindmill]

Description of what this PR is changing or adding, and why:

This PR "lands FWE" by replacing Fundamentals in the sidenav with FWE, and generally cleaning up the Get Started sidenav section. This PR seems much larger than it is because some files moved location and other files were deleted. The easiest way to see the real changes is looking at the sidenav.yml file.

• Deletes Fundamentals
• Updates sidenav to point to the new /learn section
• De-dupes Setup Flutter and Install Flutter
• Simplifies 'Get Started' section by only revealing the most important pages for /install and /learn, and defers the other options to their respective Index pages
• Deletes /get-started folder altogether (moves flutter-for to its own location
• Sets up redirects for all those changes

Issues fixed by this PR (if any):

#12068

PRs or commits this PR depends on (if any):

Presubmit checklist

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• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
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[gemini-code-assist]

Summary of Changes

Hello @ericwindmill, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on a comprehensive overhaul of the site's navigation and content structure, particularly within the 'Get Started' and 'Learn Flutter' sections. The primary goal is to enhance user experience by simplifying the sidenav, consolidating related content, and ensuring smooth transitions through redirects for relocated pages. This change aims to provide a clearer and more intuitive journey for new and existing users navigating the Flutter documentation.

Highlights

• Sidenav Restructuring: The 'Get started' section in the sidenav has been significantly reorganized and simplified, consolidating installation options and streamlining learning pathways.
• Content Relocation and Deletion: Several content files related to 'Fundamentals' and 'Get Started' have been removed or moved to new locations, reflecting an updated content strategy.
• Redirects Implemented: Numerous 301 redirects have been added in 'firebase.json' to ensure that old URLs for moved or deleted content correctly point to their new destinations or relevant sections, preventing broken links.
• Consolidated Installation Guides: The installation guides have been de-duplicated and simplified, with 'Quick start' and 'Custom setup' now serving as primary entry points under the 'Install Flutter' section.

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[gemini-code-assist]

Code Review

This pull request effectively restructures the 'Get Started' section of the website, simplifying the side navigation and implementing necessary redirects. The changes are consistent with the goal of replacing the 'Fundamentals' section and moving content to new /install and /learn paths. The overall organization is much cleaner as a result.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit c69f3bd):

https://flutter-docs-prod--pr12935-ew-fwe-nav-hj37p6xp.web.app

[sfshaza2]

lgtm, once the build error is addressed (there are some internal broken links).