templates: Improve GitHub issue templates.

[sundanc]

What does this PR do, and why?

This PR improves the GitHub issue templates by adding more comprehensive sections that help users provide better context for their reports. Each template now includes:

• A "Potential challenges or limitations" section in the platform feature template to encourage contributors to think about edge cases
• An "Impact on user experience" section in the feature parity template to highlight how users are affected
• A "Logs or additional context" section in the bug report template to facilitate debugging
• A "Frequency of occurrence" section in crash reports to help prioritize issues
• A "Documentation updates" section in the PR template to remind contributors about documentation needs

These additions help gather more complete information upfront, making it easier for maintainers to understand, prioritize, and address issues.

External discussion & connections

• Discussed in #zulip-terminal in template-improvements
• 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 - Visual changes
• Manually - Behavioral 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