docs: Improved docs readability for age

[kvanzuijlen]

What this PR does / why we need it: I tried improving the docs a bit for people using age encryption with ArgoCD. Before it looked like HELM_SECRETS_LOAD_GPG_KEYS should be configured both for GPG and age. I tried to make it more clear that for age it should be SOPS_AGE_KEY_FILE, by adding examples for age.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• [ ] the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (dcea153) 86.56% compared to head (00f9080) 86.56%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #422 +/- ## ======================================= Coverage 86.56% 86.56% ======================================= Files 23 23 Lines 856 856 ======================================= Hits 741 741 Misses 115 115 ```

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[jkroepke]

Thanks!