Fix documentation references to non-existent API

[KalimeroMK]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

What does this PR do?

Brings the guide back in line with the code on master. Every item below was checked against src/ and reproduced by running the package, not read off the prose.

• Yiisoft\Queue\Message\EnvelopeInterface does not exist — envelopes extend the abstract Envelope.
• getMetadata() does not exist — MessageInterface declares getMeta(). It appeared in two copy-paste-ready snippets (performance-tuning.md, best-practices.md).
• Envelope::fromPayload() was documented as a second way to create an envelope; it is final and always throws LogicException.
• The push pipeline section documented PushRequest, withAdapter() and Yiisoft\Queue\Exception\AdapterNotConfiguredException. None exist: processPush() takes and returns a MessageInterface, and the adapter is fixed when the producer is built.
• envelopes-metadata-internals.md described an envelope stack kept under ENVELOPE_STACK_KEY ("envelopes") and rebuilt by the serializer. There is no such key or logic anywhere in src/. The page is rewritten to describe what the code does: envelopes merge into meta and flatten (getMessage() returns the innermost message), serialization carries type/payload/meta only, and the wrapper classes are not rebuilt — values are read back through IdEnvelope::fromMessage() / DelayEnvelope::fromMessage().
• configuration-manual.md mapped DownloadFileMessage::TYPE to FileDownloader, which is the handler's injected dependency everywhere else in the docs; the handler is RemoteFileHandler.

The rewritten page also warns against registering an Envelope subclass in the message class resolver: unserialize() then calls Envelope::fromPayload() and dies with a LogicException.

Note on #338

The push pipeline section is written against master. If #338 lands and reintroduces PushRequest, that section needs another pass — happy to follow up either way.

[KalimeroMK]

Second pass, after re-reading the files the first commit touches:

• the overview still told push middlewares they can "switch queue, choose a different adapter", 55 lines above the section this PR replaced with "the adapter is not part of the pipeline" — processPush() receives a MessageInterface and returns one, so neither is reachable
• FailureHandlingRequest is described as carrying "the queue instance"; it holds string $queueName
• the "Avoid deep envelope stacking" tip warns about per-layer overhead, while Envelope::__construct() flattens — getMeta(), getType() and getPayload() cost the same at any wrapping depth
• new Message(...) appears in 15 examples across best-practices.md and performance-tuning.md; Message is abstract and holds only metadata, so those mean GenericMessage, whose (string $type, $payload) signature the examples already match