docs: guides section

[hejsztynx]

Summary

Added the Guides section to the docs. Aside from basic tutorials that taught general features in the Rich text formatting section, this section present 3 tutorials:

• building user and channel mentions - similarly as it is in the main example app in the repo
• building an emoji picker using mentions - shows that mentions are pretty powerful and can be used in more interesting situations
• contextMenuItems - no live demo, as it is not available on web

I tested the same code in an RN app - everything works nicely, the code is portable

[Copilot]

Pull request overview

Adds a new Guides section to the Docusaurus docs with interactive/tutorial-style pages demonstrating more complete, real-world patterns (mentions pickers, emoji picker via mentions, and native context menu customization), replacing placeholder guide pages.

Changes:

• Added two new interactive example components for guides: user/channel mention picker and emoji picker.
• Added new MDX guide pages for user/channel mentions and emojis, wired to live interactive examples (Preview/Code).
• Replaced the placeholder “Custom context menu” page with a full native-only guide and removed other placeholder guide stubs.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
docs/src/examples/MentionOnlyEditor.tsx	New interactive example implementing @/# mention pickers for the Guides section.
docs/src/examples/EmojiEditor.tsx	New interactive example implementing an emoji picker using mention mechanics (:).
docs/docs/guides/user-and-channel-mentions.mdx	New guide page explaining how to build user/channel mentions with a picker + live example embed.
docs/docs/guides/emojis.mdx	New guide page explaining emoji insertion via mentions + live example embed.
docs/docs/guides/custom-context-menu.md	Replaced placeholder with a full guide and example snippet for contextMenuItems (native-only).
docs/docs/guides/mention-only-input.md	Removed placeholder stub page.
docs/docs/guides/emojis.md	Removed placeholder stub page (replaced by .mdx).
docs/docs/guides/chat-input-with-images.md	Removed placeholder stub page.

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