docs: add custom element page

[mdanilowicz]

Description

closes #505

Type of change

ToDo's

Screenshots (if applicable)

Additional context

[vercel]

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[codspeed-hq]

CodSpeed Performance Report

Merging #840 will not alter performance

_{Comparing feat/GH-505 (1f125e8) with main (18d8528)}

Summary

✅ 9 untouched benchmarks

[BrocksiNet]

The main idea of that issue was to have a step by step guide from admin/backend to frontends component. If I would have no idea about admin/frontends this new page would not be helpful to me. I would ask myself what is the key fact I am learning here on this page and what is the difference to "Overwritting CMS" Page. Also there is no link to admin docs about how to create a CMS Element.

[mdanilowicz]

The main idea of that issue was to have a step by step guide from admin/backend to frontends component. If I would have no idea about admin/frontends this new page would not be helpful to me. I would ask myself what is the key fact I am learning here on this page and what is the difference to "Overwritting CMS" Page. Also there is no link to admin docs about how to create a CMS Element.

ok, got it I'll change it

[BrocksiNet]

Custom Element is missing in main CMS Page in Docs.