CI: Add misspell spelling checker tool and CI verification

[dongjiang1989]

Summary

• Add misspell tool integration for detecting and fixing spelling errors in markdown files
• Add spelling-update (fix) and spelling-verify (check) Make targets, following the same pattern as existing toc-update/toc-verify
• Add spelling-verify CI job in GitHub Actions to enforce spelling checks on PRs
• Rename toc-update job to toc-verify in CI for naming consistency
• Fix make help awk regex to support aliased target names

Changes

File	Description
Makefile	New MISSPELL_BINARY, spelling-update, spelling-verify targets; updated binary install rule; fixed help regex
scripts/tools.go	Add github.com/client9/misspell/cmd/misspell import
scripts/go.mod / go.sum	Add misspell v0.3.4 dependency
.github/workflows/check.yml	Add spelling-verify job; rename toc-update job to toc-verify

Which issue(s) this PR fixes:

Fixes #

Special notes for reviewers:

Add misspell spelling checker tool and CI verification

[dongjiang1989]

Use:

$ make help

Usage:
  make <target>

  help                       Display this help.
  clean                      Remove all generated files and directories.

Generating
  toc-update                 Update table of contents in markdown files.
  spelling-update            Fix spelling errors in markdown files.

Checking
  toc-verify                 Verify table of contents are up to date.
  spelling-verify            Verify spelling in markdown files.

Dependencies
  tidy                       Tidy Go module dependencies.