Backport the gaffer projections docs to 26.1

[George-Payne]

Backports #5710 and #5711 to release/v26.1. Neither cherry-picked cleanly, so the two commits are rebuilt here against this branch's shape.

• tutorial.md and intro.md - master's content verbatim. This branch has the projections engine V2 page, so the V2 references carry over unchanged.
• custom.md - the same three edits as on master. The debugging section replaced here is the browser walkthrough this branch still carries, rather than the short Gaffer paragraph master had by the time Mention the projection tooling in the projections docs #5711 landed.
• Screenshots - the four projections screenshots are unreferenced once those sections go, so they are deleted. Master has no projections/images/ directory at all.

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[qodo-code-review]

PR Summary by Qodo

Backport gaffer-based projections documentation to v26.1

📝 Documentation 🕐 20-40 Minutes

AI Description

• Backport projections tutorial rewrite to use gaffer for local runs and deployment.
• Document projections tooling (gaffer, VS Code extension, testing library) and references.
• Remove obsolete projections UI/debugging screenshots after dropping browser-based walkthrough.

Diagram

graph TD
  U["Developer"] --> D["Projections docs"] --> G["Gaffer CLI"] --> DB[("KurrentDB")]
  G --> F[/"Event fixtures"/]
  VS["VS Code extension"] --> G

  subgraph Legend
    direction LR
    _doc["Doc page"] ~~~ _file[/"Fixture file"/] ~~~ _db[("Database")]
  end

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High-Level Assessment

For a release-branch backport, rebuilding the two doc commits to match v26.1’s existing projections pages is the right approach. The PR keeps master’s narrative (gaffer-first workflow) while adapting the one branch-specific divergence (replacing the older browser-debugging walkthrough) and cleans up now-unreferenced screenshots.

Files changed (3) +310 / -126

Documentation (3) +310 / -126

custom.mdAdd gaffer callouts and replace browser debugging walkthrough +13/-25

Add gaffer callouts and replace browser debugging walkthrough

• Adds a tip pointing readers to gaffer for local projection execution and deployment. Replaces the legacy browser-based debugging walkthrough with guidance on logging and stepping through projections using gaffer/VS Code (DAP), removing references to the deleted screenshots.

docs/server/features/projections/custom.md

intro.mdClarify user projection creation paths and add a tooling section +16/-2

Clarify user projection creation paths and add a tooling section

• Refines the description of how user-defined JavaScript projections are authored/created (API, client libraries, embedded UI). Adds a new Tooling section describing gaffer, the VS Code extension, and the testing library with links.

docs/server/features/projections/intro.md

tutorial.mdRewrite projections tutorial around gaffer local development and deployment +281/-99

Rewrite projections tutorial around gaffer local development and deployment

• Replaces the HTTP-API/UI-centric walkthrough with a gaffer-based tutorial: scaffold projections, run them locally against fixture events, then deploy to a Docker-started KurrentDB instance. Adds guidance for partitioned projections, checking deployed state, diffing drift, and handling logic changes (reset-on-logic-change), and updates next-step links accordingly.

docs/server/features/projections/tutorial.md

[qodo-code-review]

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