Resolve "Filesystem Overview in Renter README"

MERGE REQUEST

Overview

This MR moves the Siafile and Siadir modules into the filesystem and updates the READM.md files accordingly.

Example for Visual changes (ie Screenshot)

Issues Closed

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods, or updating methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to work flow
• Issue added to Sia-UI repo for new supporting features
• Changelog updated

Closes #3840 (closed)