Resolve a README's relative links against the file, not the page

A relative link in a README was resolved by the browser against the URL the README happened to be rendered at, not against the file that contains it. On /r/<repo> the link docs/getting-started.md became /r/<repo>/docs/getting-started.md, and on /r/<repo>/code it became /r/<repo>/docs/getting-started.md from a different base. Both 404. Images broke the same way.

The fix gives a document that lives in the tree its own path, and rewrites the links and images in it after parsing and before rendering:

Left alone, because they already name somewhere: a scheme (https:, mailto:), //host, a site-absolute /path, and a bare #anchor or ?query that refers to the document itself. Also left alone: content with no place in a tree — issue bodies, comments and release notes keep their relative links exactly as authored.

How

The rewrite walks the goldmark AST rather than the markdown text, so reference-style links and images are covered by the same rule, and a link inside a code fence is never touched because it is not a link node. The source view still shows the file’s bytes; only the rendered preview and the README that the overview and code page display are rewritten. .. is clamped at the repository root instead of walking out of it.

Tests

• server/web/markdown_test.go — docHref table test covering every rule, including directories, parent traversal, refs with a slash, and the destinations that must not be rewritten.
• server/web_test.go — pushes a README plus a document in a subdirectory and asserts the resolved links on the overview page, on the code page, and on the subdirectory document’s own preview; asserts the source view is unchanged.

Not in this change

Heading anchors. docs/architecture.md#using-vole-as-a-library now opens the right file, and the fragment is preserved in the URL, but the document is rendered without heading ids, so it does not scroll to the section. Adding them is a separate decision about generated ids on every rendered document (issues and comments included), where a heading named main would collide with the page’s <main id="main">. Worth its own issue rather than a rider on this one.

Closes #01M5XF52R132A88C6WEQ6H6BK6