Add view slot examples, images, and CSS architecture to v7 docs

[shagundeogharkar]

Summary

• Added view slot examples with images and code blocks to 11 component docs (conversations, groups, users, group-members, ai-assistant-chat, message-header, message-list, message-composer, incoming-call, outgoing-call, search, thread-header)
• Replaced TODO:IMAGE placeholders with actual view slot demonstrations from v6, adapted for v7 architecture
• Replaced CSS Selectors section with full CSS Architecture section in conversations.mdx
• Rewrote message-bubble-styling.mdx to match v6 structure with all message type examples
• Removed unsupported options prop from conversations docs

V7 Architecture Adaptations

• No separate CSS import (styles bundled automatically)
• CometChatAvatar/Button/Date use compound component pattern
• useLocale() hook instead of getLocalizedString
• useLoggedInUser() hook instead of CometChatUIKitLoginListener
• Plain objects for options instead of CometChatOption class
• CometChatListItem not publicly exported