v6: fix broken migration guide examples and transport docs

[trevor-scheer]

Summary

A docs review cross-checked the migration guide and package READMEs against the actual graphiql-6 source and found several that describe APIs that no longer exist. The migration guide's useDocExplorer/useHistory "after" examples pass a selector argument and destructure fields (.navStack, .items) that don't exist — both hooks take zero arguments and return the value directly, so copying the examples as written is a compile error. The graphiql and graphiql-react READMEs describe transport as "a function," which is the old Fetcher shape; it's an object with a send() method. The createTransport link in graphiql/README.md pointed at createFetcher.ts, and neither README mentioned that fetcher still works. The "Editor Theme" section documented a CodeMirror API that hasn't existed since the Monaco rewrite. The toolkit's own README never mentioned createTransport, its biggest new export. create-fetcher.md's deprecation banner linked to a create-transport.md file that doesn't exist, and (found while fixing that line) the migration-guide link right next to it was also pointing one directory too shallow.

Also added the ExecuteButton removal to the migration guide's breaking-changes notes, reworded the .browserslistrc line (the file still exists, only its contents changed to defaults), and added the v6 migration guide to graphiql/README.md's getting-started list, which already linked to it later in the doc but didn't list it up top.

Test plan

• Copy the useDocExplorer and useHistory "after" examples from the migration guide into a scratch .tsx file inside the repo and confirm they compile with no excess-argument or missing-field errors.
• In graphiql/README.md, click the createTransport link and confirm it lands on createTransport.ts, not createFetcher.ts.
• In packages/graphiql-toolkit/docs/create-fetcher.md, click both links in the deprecation banner and confirm each resolves to a real file.
• In packages/graphiql-toolkit/README.md, click the new createTransport link and confirm it resolves.
• Read the rewritten "Editor Theme" section in graphiql/README.md and confirm the editorTheme prop example matches what <GraphiQL> actually accepts (a { dark, light } pair of Monaco theme names, not a CodeMirror theme string).
• Confirm packages/graphiql-toolkit/src/create-transport/README.md does not appear in this PR's diff.

Refs: #4219

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