Add linting for HELP_CATEGORIES

[rsashank]

What does this PR do, and why?

I was working on #1544 and added a new key under a new topic_actions category. I noticed that while linting passed, the hotkeys.md file wasn’t updating correctly when I ran --fix. It turned out the issue was that the HELP_CATEGORIES dictionary wasn't updated, so I added linting for that.

• Implemented a linting check to ensure all key_category values in KEY_BINDINGS are present in the HELP_CATEGORIES dictionary.
• Added functionality to print any missing categories if they are not found in HELP_CATEGORIES.

New category	No new category

External discussion & connections

• Discussed in #zulip-terminal in topic
• Fully fixes #
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit

[rsashank]

Closed. Addressed in #1518.