Add options to change default static media directory

[coreman14]

Related Issue
#355

Proposed Changes
Added a argument to fast_app that allows the user to change the default static media folder being the root directory. Updated the documentation to reflect this change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I am aware that this is an nbdev project, and I have edited, cleaned, and synced the source notebooks instead of editing .py or .md files directly.

[gitnotebooks]

Found 2 changed notebooks. Review the changes at https://app.gitnotebooks.com/AnswerDotAI/fasthtml/pull/373

[jph00]

Oh good idea!