[cloudnative-pg] Split the installation readme to individual charts

[danishedb]

Currently all the charts installation instruction is inside readme file located at root folder.
Problem:
In case if we just pull the charts tar file from releases it lacks the proper installation guide per chart it has just the auto generated readme from helm docs.

Solution:

Created a template in the helm docs which shall contain the details of installation as well.
Split the individual installations inside charts directory ( Cluster , CloudnativePG and Barman Plugin)

[mnencia]

Thanks for raising this, the gap you're describing is real. The cluster chart already ships installation instructions in its packaged README because charts/cluster/README.md.gotmpl includes a "Getting Started" section before the helm-docs values table. The cloudnative-pg and plugin-barman-cloud charts don't have a README.md.gotmpl, so make docs falls back to the default helm-docs layout and the install snippets only live in the root README.md, which isn't part of the chart tarball.

Easiest path is to follow the cluster chart pattern: add a README.md.gotmpl to each of the other two charts with a short installation section above {{ template "chart.requirementsSection" . }} and {{ template "chart.valuesSection" . }}, then regenerate with make docs.

While you're at it, please also trim the root README.md down to a generic overview that points at the per-chart READMEs. The install snippets belong in the chart that ships them, not duplicated at the repo root where they'll drift.

Go ahead.