docs(runtime): prefer ObserverRuntime examples

[Lythaeon]

Description

This PR updates the public SOF docs to prefer the newer ObserverRuntime composition style over the older run_async_* convenience wrappers.

The existing snippets were still valid, but they no longer reflected the clearest public API for new users after the lifecycle/runtime work landed in 0.11.0. This PR makes the docs match the intended runtime surface.

Changes

Detailed list of what changed:

• crates/sof-observer/README.md: rewrote the runtime quickstart and plugin/extension examples to lead with ObserverRuntime::new()
• docs/gitbook/crates/sof.md: updated the main sof crate page to show builder-style runtime composition
• docs/gitbook/getting-started/install-sof.md: updated starter snippets to use ObserverRuntime::new() and attached hosts explicitly
• docs/gitbook/getting-started/first-runtime.md: updated the progression and code samples to prefer ObserverRuntime + RuntimeSetup
• docs/gitbook/use-sof/observer-service.md: updated the observer service example to use the composed runtime style
• docs/gitbook/use-sof/observe-and-submit-service.md: updated the combined observe-and-submit example to use ObserverRuntime directly

For slice-related changes, include:

• Affected slices: documentation only
• Cross-slice communication changes (if any) and why: none
• Migration requirements (if any): none; this is a docs refresh, not an API change

Motivation

Business motivation:

• make the public docs match the cleaner API new adopters should actually copy
• reduce confusion between convenience wrappers and the preferred runtime composition surface

Technical motivation:

• ObserverRuntime is now the clearer top-level public style for runtime composition
• the docs should lead with with_setup(...), with_plugin_host(...), and run_until_termination_signal() instead of older wrapper functions

Alternative approaches considered:

• leaving the wrapper-based snippets in place because they still compile
• adding ObserverRuntime as a secondary note instead of making it the primary documented path

Scope and impact

• Affected slices: docs/GitBook and observer README
• Data/API changes: none
• Backward compatibility: fully backward compatible; docs only
• Performance impact: none
• Security impact: none

Testing

• Unit tests
• Integration tests
• Manual verification
• Performance checks (if applicable)
• Security checks (if applicable)

Commands/results:

cargo fmt --all
cd docs/gitbook && npm run build

Related issues and documentation

• Fixes:
• Related:
• Architecture docs: docs/architecture/README.md
• Relevant ARD/ADR:
• Operations/runbook updates:

Reviewer checklist

• Code follows project standards and architecture constraints
• Slice boundaries are respected (docs/architecture/ard/0003-slice-dependency-contracts.md)
• Tests added/updated and passing
• Documentation updated (README/docs/operations as needed)
• No undocumented breaking change
• Performance trade-offs documented where relevant
• Security considerations addressed where relevant

Additional notes

• The old run_async_* entrypoints are still valid convenience wrappers.
• This PR only changes which style the docs recommend first for new code.