docs: add bloblang about page with expanded literals section

[twmb]

Closes #1115

[claude]

Claude finished @twmb's task in 1m 59s —— View job

---

Review

• Gather context (PR diff, agent files)
• Validate commit policy
• Code review
• Post summary

---

Commits

LGTM

Review

This PR adds a new Bloblang "about" documentation page via the existing docs_gen template pipeline. The Go changes follow the established pattern (embed → parse → execute → write). The template is static (no {{ }} directives), which is fine and consistent with how the doc generation tool works. The expanded literals section with numbers, booleans, null, strings, triple-quoted strings, arrays, objects, and dynamic expressions is a solid addition.

1. Broken cross-reference: bloblang_about.adoc.tmpl L17 — xref:guides:bloblang/about.adoc#interpolation[interpolation] references a #interpolation anchor that does not exist in this page.

---

[claude]

Bug: This xref references #interpolation, but there is no == Interpolation section or [[interpolation]] anchor defined anywhere in this page. This will be a broken link in the rendered docs. Either add an Interpolation section to this page or update the xref to point to the correct location.

[claude]

Bug: This generated file doesn't match the template it's supposed to be generated from. Since doBloblangAbout executes the template with nil data and the template contains no Go template directives, the output should be byte-for-byte identical to bloblang_about.adoc.tmpl.

However, this line has xref:guides:bloblang/about.adoc#interpolation[interpolation] while the template has xref:configuration:interpolation.adoc#bloblang-queries[interpolation].

The generated file should be regenerated from the template (via task docs) to ensure they're in sync.

[claude]

Commits
LGTM

Review
The Go code changes follow existing patterns correctly. The generated doc file has a content mismatch with its source template.

1. Generated file out of sync with template (docs/modules/guides/pages/bloblang/about.adoc#L15): The generated file contains xref:guides:bloblang/about.adoc#interpolation[interpolation] but the template has xref:configuration:interpolation.adoc#bloblang-queries[interpolation]. Since doBloblangAbout passes nil data with no template directives, these must be identical. Re-run task docs to regenerate.

[claude]

Commits
LGTM

Review
Adds a Bloblang "about" documentation page via the docs generation pipeline: new template, embed directive, template parsing, and generation function in docs_gen/main.go, plus the generated output. Code follows existing patterns for doc generation.

LGTM