📖 Add Go section to install clusterctl

[sivchari]

What this PR does / why we need it:

I added the Go section to install clusterctl via go install.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

/area documentation

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[sivchari]

Ah, sorry. Presumably, we can't embed the some versions via ldflags option if we use go install.
So, this PR might not be necessary. Or, do we need to deal with the way to use go install ?

[chrischdi]

Ah, sorry. Presumably, we can't embed the some versions via ldflags option if we use go install. So, this PR might not be necessary. Or, do we need to deal with the way to use go install ?

An alternative would be to make clusterctl not fail when the ld flags have not been set. Afaik they are only used for the version subcommand (which should be okay) and as post run to check for new versions.

[sivchari]

I think it's prefer the way you propose. So I'd open the issue to discuss about it.