Add instructions to install pipx

[stevepiercy]

• Refactor locations of includes so that Volto aligns with Documentation

Closes #1771

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📚 Documentation preview 📚: https://plone6--1784.org.readthedocs.build/

[stevepiercy]

ping @nileshgulia1 @jackahl @danalvrz @iRohitSingh @tkimnguyen @polyester to make y'all aware of this revision to the main documentation. It is critical for the success of your trainings and avoid common pitfalls by first-timers to refer to these installation instructions.

Good luck at PloneConf! And, as always, if y'all hit any problems with these docs at any time, but especially during trainings, please let me know. Thank you!

[stevepiercy]

The failing linkcheck is temporary and can be ignored.