Adding a new sub-guide "Render Elements" to the existing "Render API" guide

Documentation location/URL

https://www.drupal.org/docs/drupal-apis/render-api →

Problem/Motivation

There is not a guide where we can list all render elements guides that will be created.
For more information, please see the parent issue: #3313535 🌱 [Meta] Improve documentation for Render and Form Elements Active .

Proposed resolution

Inside Render API guide, create a sub-guide called Render Elements with these suggested topics:
- What are Render Elements?
- What are Form Elements?
- Common properties of Render Elements
List of links for each render element guide:
-- Actions
-- Ajax
-- Button (FormElement)
-- ...
-- View
-- Weight