fix(docs): repair broken Flet Gallery links

[UpadhyayKiran]

Description

The old https://flet.dev/gallery URL returns the Flet website's 404 page after the Gallery moved to Flet Studio.

This change:

• updates the remaining README Gallery links to https://studio.flet.dev/gallery
• updates the stale Declarative Gallery link to the corresponding Flet Studio route
• adds permanent redirects for /gallery and /gallery/ so existing bookmarks and external links continue to work

The redirect uses the existing website/static/_redirects file and adds no dependencies.

Validation

• confirmed https://studio.flet.dev/gallery returns HTTP 200
• confirmed https://studio.flet.dev/gallery/declarative returns HTTP 200 and manually verified that it opens the Declarative Gallery category
• confirmed no remaining flet.dev/gallery or flet.app/gallery references
• git diff --check passes
• yarn install --immutable --inline-builds passes
• production docs build was attempted; CrocoDocs could not generate some CLI partials in the fresh Windows environment, causing Docusaurus to fail on those missing generated files. No error was related to these Gallery changes.

Summary by Sourcery

Update documentation links and site redirects to point the Flet Gallery to its new Flet Studio location.

Deployment:

• Add permanent redirects for /gallery and /gallery/ to the new Flet Studio gallery URL so existing bookmarks and external links remain valid.

Documentation:

• Update all remaining README and cookbook references to the Apps Gallery to use the new studio.flet.dev Gallery URLs, including the declarative examples route.

[CLAassistant]

All committers have signed the CLA.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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