docs: add command reference for explain and plan commands

[betegon]

Summary

The command reference documentation in docs/src/content/docs/commands/issue.md is missing detailed documentation for the sentry issue explain and sentry issue plan commands.

Current State

What exists:

• docs/src/content/docs/commands/issue.md documents list and view commands with full syntax, options, and examples
• docs/src/content/docs/features.md (lines 176-219) has a high-level overview of the Seer AI features including explain and plan
• Implementation code exists in src/commands/issue/explain.ts and src/commands/issue/plan.ts

What's missing:

• sentry issue explain - no command reference with syntax, options table, and detailed examples
• sentry issue plan - no command reference with syntax, options table, and detailed examples

Requested Changes

Add documentation for both commands to docs/src/content/docs/commands/issue.md following the same format as list and view:

1. sentry issue explain

   • Command syntax
   • Arguments table
   • Options table (including --json, etc.)
   • Example output

2. sentry issue plan

   • Command syntax
   • Arguments table
   • Options table
   • Example output

Reference

• Implementation: src/commands/issue/explain.ts, src/commands/issue/plan.ts
• Existing docs pattern: docs/src/content/docs/commands/issue.md
• Feature overview: docs/src/content/docs/features.md lines 176-219