docs: rename evaluators to graders

[christso]

Summary

• Renames apps/web/src/content/docs/docs/evaluators/ to docs/graders/ and custom-evaluators.mdx to custom-graders.mdx, and updates the Astro sidebar entry to "Graders".
• Replaces "evaluator(s)"/"Evaluator(s)" with "grader(s)"/"Grader(s)" across all .mdx files under the docs site (using word boundaries so "evaluation/evaluating/evaluated" prose is preserved).
• Aligns user-facing docs with the EVAL.yaml config types (llm-grader, code-grader) and follows the agentevals-research taxonomy recommendation that the scoring layer be called the "grader". No redirects added since docs are still early-stage.

Scope

• Only apps/web/src/content/docs/ and apps/web/astro.config.mjs were modified. No changes to packages/, plugins/, scripts/, or any source code — internal API names like evaluator_overrides and EVALUATOR_RESULTS_JSON are intentionally left intact in docs because they reference unchanged code identifiers.

Test plan

• Run the docs site locally (bun --filter @agentv/web dev) and confirm sidebar shows "Graders" and pages load at /docs/graders/...
• Spot-check internal links from quickstart / guides land on grader pages

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