salt: Add dedicated Ingress for MetalK8s docs

[gdemonet]

Adding a rewrite-target annotation is needed when the base path is not
/, because NGINX does not know where to find the documentation static
files.

We now always expose MetalK8s docs behind /docs, optionally appending
the configured base path for MetalK8s UI (e.g. /docs/platform).
This will avoid strange conflicts with the behaviour of the current
Ingress when running MetalK8s at a non-root path, because mixing regular
expressions and exact matches does not play well with ingress-nginx.

See: #3254
See: https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/annotations/#use-regex

[bert-e]

Hello gdemonet,

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[bert-e]

Waiting for approval

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[gdemonet]

Ideally, we would add some simple end-to-end tests checking that all exposed services continue working as desired when configuring the UI base path. We will need to postpone this task to a later improvement however, due to pressing matters.

[gdemonet]

/approve

[bert-e]

Waiting for approval

The following approvals are needed before I can proceed with the merge:

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Peer approvals must include at least 1 approval from the following list:

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• @alexandre-allard-scality

• @thomasdanan

• @gdemonet

• @TeddyAndrieux

• @lucieleonard

• @JBWatenbergScality

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[bert-e]

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[bert-e]

I have successfully merged the changeset of this pull request
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Goodbye gdemonet.