Migrate existing markdown documentation files to DocC

### Description

This issue will act as an umbrella for all issues related to migrating the existing markdown files into docc.

### Expected behavior

_No response_

### Actual behavior

_No response_

### Steps to reproduce

_No response_

### Swift Package Manager version/commit hash

_No response_

### Swift & OS version (output of `swift --version && uname -a`)

_No response_