Install docs fixes

[stefanprodan]

Fix fluxctl install command, replace --git-path with --git-paths.
Fix podinfo example, replace PODINFO_UI_COLOR with PODINFO_UI_MESSAGE.
Fix Kustomize install, add namespace.yaml to resources.

[hiddeco]

Be aware that this won't be published to the stable documentation until the next release.

[stefanprodan]

@hiddeco can we pin the stable to a master commit instead of a tag? The current fluxctl install command is broken 😢

[hiddeco]

@stefanprodan not possible, stable is magic from RTD based on published semver (git) tags.

[stefanprodan]

I would switch back to master then...

[2opremio]

Fix fluxctl install command, replace --git-path with --git-paths.

Oh dear. It sucks that it's inconsistent with fluxd, sorry about that. Since it's already released, should we simply accept both?

[stefanprodan]

@2opremio yes I think accepting both would be best.