Update docs to have sections and add existingSecret for mariadb, postgresql, and redis

[jessebot]

Pull Request

Description of the change

Adding exsitingSecret parameters for MariaDB, PostgreSQL, and Redis.

Also updating docs to:

• make it easier to link to sections of the configuration by separating out:
• database config params
  • added new exsitingSecret params for both mariadb/postgres
• persistence config params
• metrics config params
• add redis existingSecret params
• update supported versions of k8s (we can't support 1.10-1.19 as they are not even in support by k8s anymore)
• update/add links to bitnami helm charts (they use main instead of master branch now)

Benefits

Gives README a much needed face lift to make it easier to find stuff. Allows people to use existing secrets by default for all our services.

Possible drawbacks

🤷

Applicable issues

• closes Feature: existingSecret for redis and mariadb deployment #350

Checklist

• DCO has been signed off on the commit.
• Chart version bumped in Chart.yaml according to semver.
• (optional) Variables are documented in the README.md

[jessebot]

The changes to README are big enough that it might make sense to just look at it here:
https://github.com/nextcloud/helm/blob/7d490927c84f95f450e8c847c1205f711e60e396/charts/nextcloud/README.md

[jessebot]

@tvories or @provokateurin could you please have a look at this when you have some free time? It would help in making the docs more navigable and adding in the some commonly asked for features of existing secrets for external charts. The doc update is in this PR as well because we need to start separating out the docs to point to exact things in it, at least until my onboarding calms down to my current company and I can dedicate some time to working on a proper doc site for the helm chart (because it's a massive chart, and the readme will eventually get too big).

[Nils98Ar]

@jessebot What is the status of this?

[provokateurin]

It just needs a rebase, but then it can be merged since I approved the changes.

[jessebot]

Sorry for the delay. I've been really busy at work. Just fixed the helm chart version, and after the checks run, I will merge this.

[Nils98Ar]

Sorry for the delay. I've been really busy at work. Just fixed the helm chart version, and after the checks run, I will merge this.

@jessebot Thank you!