fix config links and apiVersion of example

[jonmoter]

What does this PR do?

#435 renamed docs/configuration.md to docs.configuration.v1alpha1.md, but that broke links to that page. This fixes the links.

I also noticed that the example manifest in configuration.v2alpha1.md still referred to v1alpha1, which I assume is a copy-paste typo.

Motivation

I clicked on the "Configuration" link in the README and it took me to a 404 page.

Describe your test plan

n/a, doc change only

[jonmoter]

Another pair of fixes to the configuration docs:

1. In v2alpha1, the credentials parameters now exist under global, but the example didn't show that.
2. In both example config pages, the text says it's setting agent.image.name, but that's not set in the example manifest right below it. Changed that to set the clusterName.

[codecov-commenter]

Codecov Report

Merging #590 (aa0a0be) into main (d295739) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #590   +/-   ##
=======================================
  Coverage   58.51%   58.51%           
=======================================
  Files           3        3           
  Lines         135      135           
=======================================
  Hits           79       79           
  Misses         43       43           
  Partials       13       13           

Flag	Coverage Δ
unittests	58.51% <ø> (ø)

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[jonmoter]

Okay, I've updated the change so I'm no longer linking to v2alpha1 off the main README.md