feat: URLs with no index.html suffix

[jhult]

Fixes #1251

[output.html]
no-html-extension=true

**no-html-extension:** When set to `true`, generates clean URLs without `.html` extensions. All pages are written to directory-based paths (e.g., `chapter/index.html` instead of `chapter.html`).

Special pages are placed in `_print/`, `_toc/`, and `_404/` directories.

All internal links use trailing-slash format (`chapter/` instead of `chapter.html`).

Works with all major static hosting providers (GitHub Pages, Netlify, Cloudflare Pages) and the built-in `mdbook serve` command.

**Note:** Chapter paths starting with `_print`, `_toc`, or `_404` will cause a build error.

Defaults to `false`.

Vaguely related PRs: #2570, #2993, #3028, #3034, #3038

[bens-schreiber]

Certainly fixes my issue #3028 . Would love to get this in.

[jhult]

@ehuss or @Dylan-DPC, I think many people would appreciate this being merged. I'd love any feedback you have to share.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.