Extend documentation with a guide for plan resolvers (#244)

[msotnikov]

Description

Addresses graphile/crystal-pre-merge#244 — general recommendations when writing plan resolvers.

Documentation:

• Added a new "Best practices" page (grafast/website/grafast/plan-resolvers/best-practices.md) covering the four recommendations from the issue:
  1. Deep argument extraction — use getRaw(["path", "to", "value"]) or $-prefixed destructuring instead of shallow extraction followed by lambda transforms
  2. Prefer custom steps over lambda — comparison table, guidance on when lambda is appropriate vs when to create a step class, worked example of a custom step
  3. File-scoped lambda callbacks — why inline anonymous functions defeat deduplication, with do/don't examples
  4. Avoid try/catch in plan resolvers — why imperative error handling doesn't fit the declarative model, with examples using inhibitOnNull() and trap() instead
• Added a "When to use something else" section to lambda.md pointing users toward loadOne/loadMany, custom steps, and sideEffect() before they reach for lambda
• Added a tip callout linking to the best practices page from the top of plan-resolvers/index.mdx

Runtime warning:

• In dev mode (NODE_ENV=development), lambda() now emits a console.warn when called with an anonymous (unnamed) callback function, since such callbacks cannot be deduplicated. The warning fires once per
  unique callback reference (tracked via WeakSet).

Performance impact

No impact in production. In development mode, a single WeakSet lookup is added per lambda() call (negligible).

Security impact

None.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

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