Improve documentation on choice of repository / persistence configuration when using helm charts

[planetf1]

The odpi-egeria-lab & egeria-base helm charts both support persistence

• odpi-egeria-lab

Intended for demos/tutorials
defaults to inmem repository (option in config notebook to use graph)
persistence not enabled at the k8s level (setting in values.yaml to enable)

• egeria-base

Intended as a quick-start for a simple egeria environment
uses graph repository
persistence enabled by default at the k8s level

These design choices, and settings to control their configuration, need clearer explanation. There is already some content in the values.yaml of each chart, but improving the docs in this repo should clarify the background/intent

We can also point to xtdb as the example of a better connector to use. This isn't yet supported by either chart, but odpi/egeria-charts#125 has been opened to implement this