docs: propose Telegram MessageEntity rich messages

[codeaholicguy]

Summary

• Add a design document for migrating Telegram rich messages from HTML parse mode to plain text plus Bot API MessageEntity spans.
• Cover UTF-16 offsets, entity nesting constraints, links/styles/code/blockquote/list behavior, entity-aware chunking, fallback behavior, rollout phases, affected files, and test plan.
• Keep this PR design-only with no production implementation changes.

Validation

• git diff --check

Risks

• Design-only PR; implementation risks remain around UTF-16 offset correctness, Telegram entity constraints, and chunk-boundary behavior.