docs: assign categories (genera) to rpc commands

[ShahanaFarooqui]

Assign categories to each rpc command which can later be used for different purposes like rune restrictions, filtering, etc.

• Define the list of categories
• Assign all possible/most fitting categories to all RPC commands
• Remove category, description, and verbose from json_command
• Plugins should either be able to use pre-defined categories or register a new category if required.

Reference:
CATEGORIES.txt

[ShahanaFarooqui]

Closing this issue after deciding to only add readonly category for now. We might add more categories depending upon user personas in future but they will not be dependent upon fine grained categories described in this issue.

Relevant issue: #7485