docs: add metrics-operator architecture

[rakshitgondwal]

Fixes #1049

I have made the following changes in this PR:

• Added the documentation for metrics operator architecture.
• Separated metrics-operator, lifecycle-operator, KLT scheduler in three different folders by modifying architecture/components/_index.md

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[StackScribe]

Line 11... github won't let me comment directly on that line, but I wonder if this is two components or three. Maybe say, "...the following components" and avoid the issue?

[StackScribe]

Very nicely done! We need @thisthat and @bacherfl to review the content, including my comments, which need to be refined/corrected

[bacherfl]

Thanks @rakshitgondwal for incorporating the changes for the description of the metrics! I found two minor nitpicks, but content wise the PR looks good

[rakshitgondwal]

Due to the use of ../../../../yaml-crd-ref/metric the markdown-link-check is failing. Should we shift to https://main.lifecycle.keptn.sh/docs/yaml-crd-ref/metric/ or https://github.com/keptn/lifecycle-toolkit/blob/main/docs/content/en/docs/yaml-crd-ref/metric.md instead?

cc: @StackScribe @thisthat

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[rakshitgondwal]

Thank you for the merge!