fix: Consistent URL endings concept (#411)

[Aayushman-nvm]

Summary

Standardizes the About page to use an extensionless permalink (/about/) and fixes relative asset paths that broke when switching away from /about.html.

---

Changes

• Permalink: about.html → /about/, with a redirect_from: /about.html redirect
• Canonical URL: Replaced .html removal hacks with {{ page.url | absolute_url }}
• Asset paths: Fixed relative images/ and css/ references using | relative_url to prevent /about/images/... misresolution
• Internal links: Replaced .html links (e.g. privacy.html) with extensionless equivalents (e.g. /privacy/)

---

Note

• The scope of this PR is intentionally limited to the About page to keep the change small, isolated, and easy to review.
• The same pattern can be systematically replicated across other pages to fully address issue Consistent URL endings #411.
• Updated head.html to remove the assumption of a flat URL structure (i.e., direct .html file endings). The previous logic did not account for nested paths or directory-based URLs.
• Similar adjustments may be required in other templates where this flat URL assumption exists.

---

Why

• Removes flat-file permalink assumptions
• Compatible with permalink: pretty and Hugo's default URL structure
• Reduces migration friction and improves long-term URL hygiene

---

Testing

• /about/ loads correctly
• /about.html redirects to /about/
• All assets and internal links resolve without issues