docs: GKE - fix some indentation, specify bash code segments

[ti-mo]

For #13627.

Does what it says on the tin. The content of the GKE getting-started guide itself is still 100% correct as far as I can tell.

There's still some weird indentation going on in the connectivity test block, but looks like that's caused by the use of only, which requires the subsequent parsed-literal block to be indented. Haven't found a way to unindent the rendered text, but that's super minor.

[pchaigno]

I'm not seeing any difference in the rendered version before and after this PR. Should I be looking at something else?

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[ti-mo]

I'm not seeing any difference in the rendered version before and after this PR. Should I be looking at something else?

Nope, this is just about the indentation in the .rst file. For some reason, the blocks I marked as bash code were indented with 3 spaces instead of 4 everywhere else in the document. 🤷

[joestringer]

Nope, this is just about the indentation in the .rst file. For some reason, the blocks I marked as bash code were indented with 3 spaces instead of 4 everywhere else in the document. shrug

@ti-mo weirdly 3-space indentation is very common in the tree for sphinx code, and at least personally I've found this to more reliably provide the indentation that we want (with the rendering we want).