docs(user-guide): add search in chats guide

[kostiantyn-pshenychnyi1]

Summary

Adds documentation for the new Search in Chats feature (EPMCDME-11633), which lets users find chats and folders by name from the chat sidebar.

Changes

• Added docs/user-guide/assistants/search-chats.md — new guide covering how to open the search panel, browse recent chats, search by name, and clear a search
• Added two screenshots to docs/user-guide/assistants/images/
• Updated sidebars.ts — added search-chats to the "Organizing and Managing Communication with Assistants" category
• Updated group-chats.md pagination to link back from search-chats
• Added 2 FAQ entries for chat search

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation