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.

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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/)

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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.

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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

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Testing

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

[MakisH]

@Aayushman-nvm I don't disagree with the need for some of the changes, but this PR touches too many different things at once, for which reason there are already conflicts.

Instead of closing this, I suggest selecting one of the aspects (the most important, in your opinion), and focusing the PR to that aspect. There is no reason to touch all of these now. Please also don't touch the formatting of lines you are not changing for the needs of the PR.

Make sure that the "files changed" view only includes the intended changes: https://github.com/precice/precice.github.io/pull/827/changes

[precice-bot]

This pull request has been mentioned on preCICE Forum on Discourse. There might be relevant details there:

https://precice.discourse.group/t/introduction-website-modernization/2852/1