Add codespell spell-check to CI

## Context

The repository has no automated spell-checking today. Typos in docs and code comments are caught only by manual review and one-off contributor PRs (for example the recent docs fix in https://github.com/aws-samples/sample-genai-on-eks-starter-kit/pull/196, which corrected 8 misspellings across 5 files).

## Proposal

Add a GitHub Actions workflow that runs [codespell](https://github.com/codespell-project/codespell) on every PR, so common misspellings in docs and source are flagged automatically instead of relying on human review.

## Scope and config

- Scan Markdown docs and source files.
- Skip lock files (`package-lock.json`, `*.lock`), the generated `site/` output, and any vendored directories.
- Maintain an `ignore-words-list` (via `.codespellrc` or `pyproject.toml`) for domain terms codespell false-positives on (product and tech names such as Karpenter, Langfuse, vLLM, Neuron).

## Open questions

- Blocking (fail the check on a hit) vs advisory (annotate only)?
- Add a pre-commit hook alongside CI, or CI-only?

This is a follow-up to the docs-typo cleanup and would prevent that class of issue going forward.

Start by reviewing the existing GitHub Actions workflows and repository paths, then run codespell to identify the files and domain terms that need configuration. Add the workflow and either .codespellrc or pyproject.toml, excluding package-lock.json, *.lock, site/, and vendored directories; done means every PR runs the check with the agreed blocking or advisory behavior.

Written by the indexing model from the issue text.