Support 'Work Experience' in CV rendering; simplify RenderCV workflow and docs

[leigeng1997]

Motivation

• Ensure entries under a Work Experience section are treated together with Experience and Volunteer when rendering the CV so work roles render in the main experience block.
• Simplify the GitHub Actions job that invokes RenderCV by removing the required external settings lookup so rendering can use defaults or internal config.
• Fix minor data and documentation issues in the CV data and customization docs for clarity.

Description

• Update _layouts/cv.liquid to collect cv.sections['Work Experience'] into the combined experience list, render the heading as Work Experience when present, and skip Work Experience when iterating other sections.
• Change .github/workflows/render-cv.yml to stop searching for a settings.yaml/settings.yml file and call rendercv render $cv_file without --settings, while keeping install, artifact upload, and push steps.
• Escape the underscore in the CUSTOMIZE.md table of contents entry by changing _config.yml to \_config.yml for correct rendering.
• Tidy _data/cv.yml by removing an empty summary field and adding area values for high/middle school entries along with minor whitespace cleanup.

Testing

• No automated tests were run for this change.

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Codex Task