es/v2: Port docs

[timostamm]

As a follow-up to #217, #215, #219, this updates the remainder of the Node.js and Web docs for the upcoming version 2.

Mechanical changes:

1. rename createPromiseClient to createClient
2. remove usage of the protoc-gen-connect-es plugin (protoc-gen-es also generates service schemas now) including generated SDKs
3. remove .js extensions in imports (the default for the plugin option import_extension changed to none, so we don't generate them anymore)
4. remove v1 pins in installation instructions
5. update links to the Protobuf-ES docs
6. replace usage of new for Protobuf messages with object literals or calls to create()

Breaking changes in Connect-ES:

1. call out that the gRPC transport requires HTTP/2 now
2. remove the "credentials" options from @connectrpc/connect-web transports and explain how to do the same with fetch
3. interceptors can no longer set fetch options - remove section
4. call out Parcel/Metro config requirement for subpath exports in FAQ

Improved compatibility:

1. "Supported browsers & frameworks" now simply links to examples-es, only one example for react native polyfills remains.
2. SSR docs are simplified, also thanks to the recent Svelte 5 and Next 15 releases. V2: Upgrade to Svelte 5 examples-es#1971 V2: Upgrade to Next.js v15 examples-es#1973

Drive-by changes:

1. use "Protobuf" with uppercase P
2. call out that HanderContext.signal also triggers on RPC completion
3. call out jsonOptions for transports
4. removed "Roll your own client" - not sure this option has seen much traction

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[smaye81]

Do you think it's worthwhile to link to our custom-client example here? https://github.com/connectrpc/examples-es/tree/main/custom-client? I know it's been a question in public Slack a couple times.

[timostamm]

I must have missed the cases in public slack, or I forgot. Maybe it's better to keep this section then.

The custom client example hasn't been updated yet. Taking a note to restore this section once it's updated.

[timostamm]

Restoring in #224.