DOCSP-53340: Add converted .md docs to source code repo /docs-sdk dir for community access

Jira: https://jira.mongodb.org/browse/DOCSP-53340 & https://jira.mongodb.org/browse/DOCSP-53526 ## Description Updated `docs/` dir to house the converted C++ 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) - External links were preserved - Links to API references were removed since they pointed to `mongodb.docs` domain - Links to MongoDB, Atlas, or other Docs pages were removed Updated main `README.md`: - Point "Documentation" section links to local repo dirs