Add Sphinx tab renderer to documentation

[phlax]

Description

Where docs diverge - eg:

• instructions for different platforms
• providing configs in different formats
• code snippets for different languages

...it would be really useful to have tab rendering

refs

• Envoy documentation should be Windows friendly #13190 (comment)
• https://github.com/executablebooks/sphinx-tabs

[davinci26]

@phlax do you have any place in mind in the docs that we could use it today?

I can add it as an example usage in my PR.

[phlax]

i had a look through the docs - there are places where we could use this, but i dont see any where we could use it without changing quite a bit of docs.

One example would be the clone instructions in the sandbox examples - this could be split between ssh/https - but im also wondering if we should put the examples intro docker instructions to an rst include - so its probs out of scope of this ticket. (edit: see comment below #13213 (comment))

i also have another ticket im looking at #9662 - where i think it would be very helpful to have this in place

[phlax]

ref (regarding building docs): #13229

@davinci26 ^^

[phlax]

i dont see any where we could use it without changing quite a bit of docs... so its probs out of scope of this ticket.

@davinci26 i added #13230 which once landed will make it easy to update this example, ie

split https://github.com/envoyproxy/envoy/pull/13230/files#diff-ba8b9695d4b21caa10303d3c9be7aa9dR9 between ssh/https