fix(docs): format markdown append writes

[eric-x-liu]

Summary

• allow gog docs write --append --markdown to append converted Google Docs formatting instead of raw Markdown text
• reuse the existing Docs API Markdown parser/formatter for append writes while preserving Drive-based Markdown replacement
• add a regression test and README example for formatted Markdown append

Root Cause

docs write --markdown only supported full replacement through Drive's Markdown conversion path. Append writes used Docs API InsertText, which inserts Markdown syntax literally unless gogcli converts formatting into Docs API requests first.

Fixes #272

Tests

• go test ./internal/cmd -run 'TestDocsWrite|TestMarkdownToDocsRequests' -count=1
• git diff --check