feat(helm): add image registry value for memcached and memcached-exporter

[MagnusRef]

What this PR does / why we need it:

Adds Docker image registry value for memcached and memcached-exporter. This is done to be able to specify custom registries for memcached and memcached-exporter.

Also adds some missing reference descriptions for memcached-exporter image values.

Which issue(s) this PR fixes:
Fixes #16197

Special notes for your reviewer:

First time contributing, so sorry if anything is missing. :)

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

All committers have signed the CLA.