Writing documentation for the web page for an e-commerce application

thank you for looking into this question and for the help.

My task is to provide a detailed documentation about very important web pages in our e-commerce application.

For example, I need to provide an detailed information about the home page with the following information,

1. Detailed information about the Blocks and sections displayed on the various layouts of the page.
2. Critical systems that are involved to display the blocks/sections.

Unfortunately I am very poor in the documentation of the beauty stuff (technical part is ok) and never done this before. I would thank you very much if you could suggest how to do it or provide any references to it to take off.

Thank you for all the help.