words and fix live samples

[yin1999]

Description

This PR has removed the macro calls from example code, replace 'widgets' with 'controls',
and replaces a broken link to 'px to em conversion' web page.

Also using Live example to split the live sample code and the example code.

[github-actions]

Preview URLs

• /en-US/docs/Learn/Forms/How_to_build_custom_form_controls

External URLs (4)

URL: /en-US/docs/Learn/Forms/How_to_build_custom_form_controls
Title: How to build custom form controls

• https://addons.mozilla.org/fr/firefox/addon/noscript/ (1 time) (Note! This may be a new URL 👀)
• https://chrome.google.com/webstore/detail/scriptblock/hcdjknjpbnhdoabbngpmfekaecnpajba (1 time) (Note! This may be a new URL 👀)
• https://www.apple.com/accessibility/vision/ (1 time) (Note! This may be a new URL 👀)
• https://www.nvaccess.org/ (1 time) (Note! This may be a new URL 👀)