[DX-955] Add use-case based SDK references across Pub/Sub pages

[sacOO7]

• Fixes https://ably.atlassian.net/browse/DX-955
• During chat-poc, LLM suggested to use pub-sub SDK to build chat app instead of core chat SDK.
• Pub/Sub documentation pages currently frame core primitives (presence, annotations, channels) as the primary way to build chat features like reactions, typing indicators, and chat rooms — without mentioning that the Chat SDK and Spaces SDK provide purpose-built APIs for these use cases. This misleads developers into building from scratch on Pub/Sub when dedicated SDKs exist. Additionally, several pages contain broken anchors, legacy textile syntax, inconsistent naming, and stale notices.
• Clarified that Pub/Sub should be used for custom features not covered by purpose-built SDKs.
• Added a link to the product guidance page for choosing the right Ably product.

Changes

SDK cross-references (prevent misguided implementations):

• Add Chat SDK/Spaces SDK asides on annotations, presence, presence-occupancy overview, Pub/Sub
  basics, data streaming guide, and dashboards guide pages
• Link inline mentions of "chat rooms", "typing indicators", "avatar stacks", and "emoji reactions"
  to the appropriate SDK docs

Broken links fixed:

• /docs/channels#publish → /docs/pub-sub#publish in push FAQs (2 instances)
• /docs/storage-history/storage#persistence → /docs/storage-history/storage#all-message-persistence
  in dashboards guide
• /docs/metadata-stats/stats#live → /docs/metadata-stats/stats#app-meta in data streaming guide
• Textile-style links ("text":/path) → MDX links ([text](/path)) in channels index, plus typo fix

Naming and consistency:

• Replace chatroom/private:chatroom channel names with my-channel/private:my-channel across
  channel states (~20 instances) and Pub/Sub advanced (~20 instances) to avoid implying Pub/Sub
  channels are for building chat
• Fix "PubSub" → "Pub/Sub" in Chat message reactions page
• Replace placeholder API keys (your-api-key, YOUR_API_KEY_NAME) with template variables
  ({{API_KEY}}, {{API_KEY_NAME}}, {{API_KEY_SECRET}}) in both Pub/Sub guides

Stale notice:

• Wrap plain-text subscription filters preview notice in proper <Aside data-type='public-preview'>
  block for consistency