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

[na-na-hi]

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.

[guidocella]

Good idea.

• Shouldn't stop be next to quit?
• Should ab-loop be under playback control? Options has also a Playback control subsection and --ab-loop-{a,b} is there. Though it would no longer be next to ab-loop-dump-cache and ab-loop-align-cache.
• Maybe group af vf af-command and vf-command under Filter commands?

[na-na-hi]

• Shouldn't stop be next to quit?

stop stops playback and doesn't quit the player.

• Should ab-loop be under playback control? Options has also a Playback control subsection and --ab-loop-{a,b} is there. Though it would no longer be next to ab-loop-dump-cache and ab-loop-align-cache.

I thought about this but ab loop commands don't control playback directly. ab-loop-align-cache also changes ab loop points like ab-loop so it needs to be moved together with ab-loop.

• Maybe group af vf af-command and vf-command under Filter commands?

Added a new section for them.