docs: Add scalability and optimization guidance

[abdulawalarif]

Summary

• Adds an Operations concept page on scaling Serverpod beyond a single process: server roles, Isolate.run for CPU work, Postgres indexes and query shape, connection pool sizing, Redis for shared cache and events, JWT auth, and streaming tradeoffs.
• States current limits honestly (no first-class read replicas or Serverpod-owned Postgres tuning), and points to database interceptors or an external proxy where needed.
• Links the page from the Operations overview and Hosting elsewhere (server roles).

Closes serverpod/serverpod#1718.

Test plan

• Open Next → Concepts → Operations → Scalability.
• Confirm the page appears in the Operations sidebar after Exception monitoring.
• Follow inbound links from Operations overview and Hosting elsewhere → Server roles.
• Spot-check outbound links: server roles, indexing, pagination, connection pool, database interceptors, JWT token manager, caching, server events, streaming, configuration reference.
• Skim for em dashes and marketing wording against STYLE_GUIDE.md.

[abdulawalarif]

Addressed the suggestions, Jamiu.
Please take another look.

[abdulawalarif]

Hey Jamiu, updated the frontmatter description to the noun-led