Add GitHub Skills learning resources

[Copilot]

Adds two interactive learning resources to the Getting Started section.

Changes

• Added skills/introduction-to-codeql - interactive course for CodeQL fundamentals
• Added skills/secure-code-game - gamified security training across multiple languages

Note: set-codeql-language-matrix action already exists at line 106 under GitHub Actions section.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Adds missing content</issue_title>
<issue_description>Add skills

Add the secure coding game

Add the set language matrix action
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Adds missing content #20

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