DOCS/man/input.rst: add sections to categorize commands

mpv has too many commands and they are unorganized in the
documentation, making it difficult to navigate.

This commit completely reorganizes the commands into several
categories to make the documentation easier to navigate.
The following categories are defined:

- Playback Control: Seeking and stepping.
- Property Manipulation: Changing property values.
- Playlist Manipulation: Playlist navigation and editing.
- Track Manipulation: Track navigation and editing.
- Text Manipulation: Text printing, expansion, escaping.
- Configuration: General configuration and related files.
- OSD: Displaying contents as OSD.
- Input and Keybind: Input configuration and key bindings.
- Execution: Execution of mpv and subprocesses.
- Scripting: Script loading and communication.
- Screenshot: Taking screenshots.
- Miscellaneous: Other commands.