docs: add GSSoC badge rate limits reference manual (#1953)

[Bhavy12-cell]

Summary

Closes #1953

What's Added

New File — docs/badge-rate-limits.md

A complete reference manual for GSSoC contributors covering:

• Rate limit policy — 20 req/min/IP enforced by checkBadgeRateLimit()
  (also corrects the BADGE_API.md which incorrectly stated 100 req/min)
• All badge endpoints — commits, streak-shield, streak with full
  parameter tables and example URLs
• Rate limit response headers — X-RateLimit-Limit, Remaining, Reset
  with example HTTP responses
• HTTP 429 handling — example code showing how to handle rate limits
• GitHub API rate limits — unauthenticated (60/hr) vs token (5000/hr)
  with signs your deployment is hitting limits
• Caching behaviour — per-endpoint Cache-Control values explained
• Best practices — safe README embedding, avoiding CI hammering
• Troubleshooting table — common symptoms, causes and fixes
• Environment variables — GITHUB_TOKEN setup for local and production
• Implementation reference — maps concepts to actual source files

Why This Matters for GSSoC

GSSoC contributors frequently ask about badge embedding and rate limits.
This manual gives them a single authoritative source instead of reading
source code to understand the limits.

Files Changed

• docs/badge-rate-limits.md — new reference manual (no code changes)

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus