ci: lint markdown frontmatter

[guoqqqi]

Summary

• Add a lint:frontmatter script that parses Markdown/MDX YAML frontmatter with js-yaml.
• Run the check in the existing lint workflow so malformed frontmatter is caught before a Docusaurus blog build.

Why

• fix(seo): optimize titles and descriptions for 50 blog posts (batch 2) #2021 introduced an unquoted : in a blog description, which made Docusaurus fail while parsing Markdown frontmatter.
• This check catches the same class of YAML frontmatter parse errors directly in CI.

Verification

• yarn install --frozen-lockfile --ignore-scripts
• yarn lint:frontmatter
• ./node_modules/.bin/eslint scripts/lint-frontmatter.js .github/workflows/lint.yml

[guoqqqi]

Additional local verification:

• Temporarily reverted the fixed description in blog/en/blog/2022/07/30/why-we-need-apache-apisix.md to the malformed unquoted value from fix(seo): optimize titles and descriptions for 50 blog posts (batch 2) #2021.
• yarn lint:frontmatter failed as expected and reported the exact file plus YAML parse location.
• Restored the quoted description and reran yarn lint:frontmatter; it passed.