Add autolinks rendering

[theletterf]

This follows up from a conversation in #2532 to add automatic link rendering from HTTPs URLs.

Files created:

• The autolink parser that detects bare https:// URLs
• 50 unit tests covering all requirements

Files modified:

• Added .UseAutoLinks() to the pipeline
• Added documentation for the autolinks feature

Features implemented:

• Valid https:// URLs are automatically converted to clickable links
• Autolinks open in a new tab (target="_blank")
• External link indicator is displayed (via existing CSS for target="_blank" links)
• URLs containing elastic.co/docs emit a hint suggesting crosslinks/relative links
• Autolinks are NOT rendered in code blocks or inline code spans
• Trailing punctuation (., ,, ;, etc.) is correctly excluded from URLs
• Balanced parentheses in URLs (like Wikipedia links) are handled correctly
• Works correctly with LLM and plaintext renderers (no changes needed)

—-

LLM usage disclosure: I've used Claude Opus 4.5 in Cursor.

[theletterf]

@elastic/docs-tech-leads This adds autolinks!

[Mpdreamz]

MarkDig supports .UseAutoLinks() natively, is there a reason we don't use it? (https only?)

[theletterf]

@Mpdreamz I think so. Do we want to support also plain HTTP (unencrypted)?

[theletterf]

@Mpdreamz After checking with Claude: there are several reasons we implemented a custom parser instead of using Markdig's .UseAutoLinks():

• Markdig's autolinks supports http://, https://, www., and email addresses. We specifically wanted only https:// URLs.
• We need to emit hints for elastic.co/docs URLs during parsing. Markdig's native extension doesn't have hooks for this.
• Markdig's autolinks creates AutolinkInline objects, which would need a separate renderer. Our implementation creates standard LinkInline objects that automatically work with the existing HtmxLinkInlineRenderer, getting target="_blank" and external link indicators for free.

If the requirements were simpler (all URL schemes, no diagnostics), we could have used:
.UseAutoLinks(new AutoLinkOptions { OpenInNewWindow = true }), but then we'd still need to custom renderer for AutolinkInline?

[github-actions]

🔍 Preview links for changed docs

• docs/syntax/links.md

[reakaleek]

Internal links need to have hx-* attributes, especially the hx-select-oob attribute.

[theletterf]

Autolinks are always assumed to be external, that's why they don't get the hx- attrib?

[theletterf]

Temporarily moving this to draft as we assess the situation regarding bare URLs in our docs (see Slack message).

[Mpdreamz]

Do we exempt /docs/api ? we might want to add an exlusion for .zip given we want to expose llms.zip and elasticsearch-data.zip soon.

[theletterf]

How would you suggest proceeding here? We could add a filtering system (potentially fragile, hard to maintain, etc.), launch Autolinks anyway despite some broken URLs and let teams fix this, or hold / cancel. See thread in Slack.