docs: added additional steps and examples to creating custom models - basic example section

[plsalvado]

Short Description

Added additional steps and examples to creating custom models - basic example section. New steps and code examples include: 1. how to register the plugins, 2. step to redeploy the hCMS app.

Relevant Links

https://deploy-preview-332--webiny-docs.netlify.app/docs/how-to-guides/webiny-applications/headless-cms/content-model-plugins

Checklist

• I added page metadata (description, keywords)
• I've added "Can I Use This?" section (if needed, e.g. if documenting a new feature)
• I added What You'll Learn at the top of the page and every item in the list starts with a lower case letter
• I used title case for titles and subtitles (in the main menu and in the page content)
• I checked for typos and grammar mistakes
• I added alt / title attributes for inserted images (if any)
• When linking code from GitHub, I did it via a specific tag (and not next / v5 branch)

Screenshots (if relevant):

New section