docs(leagues): replace stale Leagues-by-Sport table with live-endpoint pointer (Item #3)

[Mlaz-code]

Summary

Closes the last of 4 drift traps named in [[League Normalization Next Steps]] §Item #3. After this lands: api-adapters#538 + sharp-api-go#367 + sharpapi-site#87 + this PR fully address the original wiki entry.

Problem

content/en/api-reference/leagues.mdx had a hand-maintained "Leagues by Sport" table with 16 entries across 8 sports — against the atlas registry's 600+ leagues across 30+ sports.

Two issues:

1. Drift trap: required manual update on every registry addition. Coverage has been silently shrinking — soccer entries listed 5 leagues but the registry has 295.
2. Wrong canonical IDs in the docs: showed epl, la_liga, serie_a, bundesliga, mls. None are the registry's canonical slug forms (england_-_premier_league, spain_-_la_liga, italy_-_serie_a, germany_-_bundesliga, usa_-_major_league_soccer). The API does accept short forms for backward compat, but /leagues returns canonical slugs and other endpoints emit them on response payloads. Clients copying the doc example saw their epl filter "work" but their response-side eventId come back containing england_-_premier_league and assumed our docs were lying.

Fix

Replace the static table with a "Common Leagues" callout that:

• Is honest about being illustrative not exhaustive
• Uses canonical slug form for soccer (the most affected sport)
• Adds programmatic-access guidance ("never hard-code a list, call /api/v1/leagues") explicitly
• Mentions registry breadth (600+ leagues / 30+ sports) so readers understand the table is a tiny subset

Diff

content/en/api-reference/leagues.mdx: 63 lines → 15 lines (the static 8-section table → a single illustrative table + callout). Net -48 lines, +15 lines.

Test plan

• npm run build clean (full Next.js + Nextra static export, exit 0)
• Vercel preview renders the new "Common Leagues" section correctly with the Callout

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