Add inline tabs on Helping with the Developer’s Guide page for commands on different systems

[lancegoyke]

Hey again team! I was hoping to knock a few more of these out this weekend.

This PR should close a single TODO in #1196.

I did restructure the writing a bit to break up the directive "to build the devguide..." from the long paragraph before it.

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📚 Documentation preview 📚: https://cpython-devguide--1223.org.readthedocs.build/documentation/devguide/

[hugovk]

Hello! Thanks again for these!

We've also since added some JavaScript to activate the relevant tab based on the browser's operating system in #1216.

Like in that PR, please could you add this snippet near the top of the new pages with tabs?

.. raw:: html

    <script>
    document.addEventListener('DOMContentLoaded', function() {
      activateTab(getOS());
    });
    </script>

[lancegoyke]

Of course! Apologies, I had seen you mention that, but I missed the fact that it needed to be included on each page.

[hugovk]

No apology needed! Yeah, we could have loaded the JS for all pages, but for now we went with explicitly adding it to the tab pages.

On reflection, it's such a small amount of JS it's probably fine to always load it, but we can think about that later.

Anyway, I think we could also combine the Unix and macOS tabs in this one into a single Unix/macOS tab?

[lancegoyke]

Sounds good to me!

[hugovk]

Thanks!