DC-5239 Clerk + Astro Guide (#7311)

* guide created

* Optimised images with calibre/image-actions

* minor copy changes

* link removed

* language switched from astro to html for proper syntax highlighting

* updated node version

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: aidankmcalister