Docs: provider documentation is incomplete and inconsistent

[sergiopaniego]

Docs: provider documentation is incomplete and inconsistent

Two more providers are landing (#821 Modal, #841 HF) and the provider docs haven't kept pace. The reference list, getting-started, and guides are incomplete or contradict each other, and there's no documented path for adding one.

User-facing docs

• DaytonaProvider missing from reference/core.md autodoc (it's in README.md:179 but not the reference — lists disagree).
• getting-started.md:136 lists Modal/E2B as available; neither is implemented as a provider.
• getting-started.md:140 cites RFC 002 "Cloud Sandbox Providers" as settled; it's an unratified amendment.
• No runnable provider snippet in docs (examples/daytona_tbench2_*.py unlinked).
• provider= never shown in guides (concepts.md connection table omits it).
• No providers landing page in guides/.
• Install extras (aca, inspect, daytona, all) undocumented in docs/source/.

Contributor docs

• "Adding a provider" only lives in a non-ratified RFC 002 amendment; not in any CONTRIBUTING.
• Nothing keeps provider docs in sync: sync_env_docs.py is envs-only, so a new provider with no autodoc entry passes CI (that's how Daytona's got missed).

Fix (one docs PR, links #821 + #841)

1. Add DaytonaProvider autodoc; reconcile README/reference lists.
2. Fix getting-started provider paragraph + RFC citation.
3. Add guides/runtime-providers.md: provider table, install extras, provider= usage with a runnable snippet.
4. Add a short "Adding a Container Provider" section to contributing docs (subclass, autodoc entry, example, test).

Lands first so #821/#841 just add their autodoc entry instead of being cleaned up after.