Add home page in academy-theme

[CodexRaunak]

Notes for Reviewers

Fixes #183

It moves the default Academy homepage content into academy-theme so Academy repositories can inherit a shared homepage instead of duplicating the same copy in each repo.

Changes include:

• Adds a theme-level content/_index.md with shared Academy homepage content.
• Adds a theme-level layouts/index.html that renders the homepage content with {{ .Content }}.
• Add Best Practices for authors to validate and create IDs from the Academy Instructor Console.

Signed commits

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[gemini-code-assist]

Code Review

This pull request updates the .gitignore to track the content/_index.md file while ignoring other content, adds a new development landing page for the Academy, and provides a basic Hugo layout for the homepage. The review feedback suggests fixing typos and clarifying the purpose of the index page, correcting the heading hierarchy for better accessibility, and replacing a deep-link Slack redirect with a more reliable public invite link.