sagemathgh-37065: Installation guide: Use `.. envvar::` to define environment variables
    
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This turns references ``:envvar:`....` `` into hyperlinks to the place
of the definition. https://deploy-preview-37065--
sagemath.netlify.app/html/en/installation/source#environment-variables

Also improving the structure of the section, and some smaller updates.

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URL: sagemath#37065
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee

sagemathgh-37083: HTML documentation: Show preparsed doctests using inline tabs
    
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Every doctest block is decorated with tabs, by default showing the
original doctest in **Sage** syntax, and offering a preparsed, pure
**Python** version of it. The tabs are synchronized across the page.

If @kwankyu's live documentation is being built, it is offered as
another tab **Sage (live)**. When this tab is selected, it automatically
starts the Thebe/Binder; it is not necessary to find and push the "Make
live" button.

![tabs-anim](https://github.com/sagemath/sage/assets/8345221/4252ae92-
eeb7-417a-ba24-c141726f714d)

[Preview](https://deploy-preview-37083--sagemath.netlify.app/html/en/ref
erence/function_fields/sage/rings/function_field/function_field_rational
)

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Fixes sagemath#35791
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### ⌛ Dependencies

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- Depends on sagemath#37056
- Depends on sagemath#37065

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URL: sagemath#37083
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe, Tobias Diez