docs: add bloblang about page with expanded literals section

[twmb]

Closes #1115

[claude]

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

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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.

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[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.