DOCSP-53339: Add converted .md docs to source code repo /docs dir for community access

Jira: https://jira.mongodb.org/browse/DOCSP-53339 & https://jira.mongodb.org/browse/DOCSP-53526 ## Description Added new `docs/` dir to house the converted Flutter SDK docs. - All pages are converted fully to Markdown - Any references to deprecated features or products were removed (e.g. App Services and Device Sync) - Links to API references in `pub.dev.documentation` are preserved - Links to MongoDB, Atlas, or other Docs pages were removed Updated main `README.md`: - Point "Documentation" section links to local repo dirs