Add support for exporting complete documentation as a PDF

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Hi I would like to request a new feature to be added to Hextra. I think this is a very useful feature so one can keep a copy of the documentation offline and distribute it via other media.

Feature Description

Add a way to export a full Hextra documentation site as one PDF.

Problem/Solution

We currently use Astro Starlight with starlight-to-pdf to create PDF versions of our documentation for offline distribution.

We would like similar support in Hextra, ideally with sidebar order, a table of contents, internal links, images, code blocks, and CI support.

Alternatives Considered

We considered browser printing and generic PDF tools, but these usually only export one page or require custom setup.

Additional Context

Reference implementation:

https://github.com/Linkerin/starlight-to-pdf
https://aerynos.dev/aerynos.pdf

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Hi some extra context. Currently we have a pure documentation site and we would just want this functionality for the documentation. We plan to combine documentation and the main site. When we move to Hextra so it's not on separate domains.