docs: add scalable composition guide and XRD scale subresource section

[jonasz-lasut]

Closes #1076

Summary

• Adds a new content/master/guides/scalable-composition.md guide that walks through exposing the Kubernetes scale subresource on a composite resource (XRD), enabling kubectl scale, HPA, and KEDA support
• Adds a ### Scale subresource section to composite-resource-definitions.md with a cross-reference to the new guide
• Adds Vale spelling exceptions for autoscalers, KEDA, and subresource

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[jbw976]

awesome @jonasz-lasut, thanks for putting this guide together for the XRD scale subresource feature! I took it for a full test run on the preview site: https://deploy-preview-1077--crossplane.netlify.app/master/guides/scalable-composition/

the messaging/content and overall flow seems fine to me, most all my comments are around the control meta for copying/pasting, and commands to apply the resources for consistency.

Let me know what you think!

[jonasz-lasut]

Really appreciate the feedback @jbw976
I've resolved all the comments, added missing apply/get steps for composition/xrd and updated the dependencies

[jbw976]

just one more suggestion to show the packages as healthy, otherwise this is looking great!!

[jonasz-lasut]

Fixed :)