docs: add website, scenarios, and agent skill; remove stale docs

[AbeOmor]

Summary

Cleans up pg_durable docs and adds a static website, a scenarios guide, and the reusable agent skill.

Added

• .agents/skills/pg-durable-sql/ — moved the pg-durable-sql agent skill from .github/skills/ so agents (e.g. GitHub Copilot) can generate durable-function SQL.
• docs/website/ — static landing page (renamed from getting-started), deployed via GitHub Pages. Includes:
  • A "What you can build" section with use-case cards and a prominent callout to the full Scenarios guide.
  • An AI Skill section pointing to the pg-durable-sql skill.
  • An Open-source durable functions section.
  • A managed Azure HorizonDB section.
  • Repo links normalized to microsoft/pg_durable.
• docs/SCENARIOS.md — consolidated scenarios guide:
  • Part 1: Database & ETL Patterns (Scenarios 1–5).
  • Part 2: Standard Operational Scenarios → links to examples/operational-scenarios/ (autovacuum blocked, database bloat, wraparound risk, tables not vacuumed).
  • Part 3: Azure Integration Examples → links to examples/azure-functions/, examples/azure-http-domains/, examples/invoice-approval/.
• examples/operational-scenarios/ — anonymized vacuum/bloat/wraparound DB-ops scenarios (SQL + README + design doc), referenced as Part 2 in docs/SCENARIOS.md.
• .github/workflows/pages.yml — auto-deploys docs/website/ to GitHub Pages on pushes to main.

Removed

• docs/bug-bash-april-2026.md
• docs/pgai-vectorizer-durable-functions.md

Notes

• "Test pg_textsearch and update docs" was skipped for now per discussion.

[AbeOmor]

@pinodeca! When this merges, can you do the following to get the website live!

One-time setup (repo admin, in GitHub UI)
Go to the repo → Settings → Pages.
Under Build and deployment → Source, select GitHub Actions (not "Deploy from a branch").
That's the only manual step, and it only needs to be done once.

[pinodeca]

I asked Opus 4.7 to do a review mostly focused on changes you may have made to existing files. The only finding was a missing trailing newline - that pre-dates your changes.

I would go ahead and merge as-is so we get a 1st version of the website/Pages - then we can improve it as needed.