[codex] Rework homepage content

[Ridealist]

Summary

• Switch the root homepage to the new homepage layout.
• Replace the old Minimal Mistakes post-list home with profile-oriented homepage content.
• Add About Me, Interests, Learning Topics, and Research Interests sections.
• Keep the existing / URL by continuing to use the root index.html file.

Dependency

• Depends on [codex] Add homepage layout and styles #15, which adds the homepage layout and scoped homepage styles.
• Merge [codex] Add homepage layout and styles #15 first so this PR diff shrinks to the homepage content change only.

Validation

• bundle exec jekyll build
• Verified http://127.0.0.1:4000/ renders homepage.css, layout--homepage, and the new homepage sections.
• Verified category pages do not receive the homepage layout or stylesheet.
• git diff --check

Existing Sass deprecation warnings from Minimal Mistakes are still present.

Closes #9