[DOCS] Add a documentation page describing XML Schema assets and presenting a basic workflow utilizing them

[LB-ArturZieba]

Describe the issue briefly

Currently, there is no documentation page containing detailed description the .xmlschema assets, nor describing any basic workflow which would utilize them. It should be noted that .xmlschema can be created directly in Editor using the Asset Editor.

Which page(s) / section(s) are affected?

Not affected directly, but currently the only documentation page mentioning the .xmlschema asset type: https://www.o3de.org/docs/user-guide/assets/asset-types/#asset-builders