DRIVERS-2789 Convert Server Selection Spec to Markdown

[blink1073]

Please complete the following before merging:

• Update changelog.
• Make sure there are generated JSON files from the YAML test files.
• Test changes in at least one language driver.
• Test these changes against all server versions and topologies (including standalone, replica set, sharded
  clusters, and serverless).

[NoahStapp]

Approved. I didn't check for any content changes, just conversions from RST to markdown.