1.  Support installation from portable_config subdirectories.

The Readme states:

NOTE: If this command is run in an mpv installation directory with portable_config, it'll install there instead of AppData.

However, this only worked if the current directory was exactly portable_config. This PR ensures the script correctly identifies the portable root even when run from its subdirectories.

2. Fix character encoding in PowerShell.

Previously the windows install script included a →, which cannot be displayed correctly in powershell, so I replace it with ->.

3. Use a hyperlink for the custom subtitle filter example script. Unrelated to the powershell script changes, I post it here for convenience.

All these changes won't affect what has been released.