docs: update the readmes and the how-tos, add quickstart and a tutorial

[MaferMazu]

Description

This PR:

• Fix the docs (The decision 4 wasn't in any index)
• Update the Quickstart, the How-tos, and the READMEs
• Create a Tutorial

How to test

• Review the doc files here: https://github.com/eduNEXT/openedx-wordpress-ecommerce/tree/mfmz/update-doc/docs
• Or you can run an environment to see the final result of the doc. (I added screenshots)

Run an environment

• Create a python8 venv and activate it

virtualenv -p python3.8 venv
source venv/bin/activate

• Install requeriments

make requirements

• Enter in docs

cd docs

• Compile

make html

• Run the server

cd ..
make serve_docs

(By default uses port 8000)
or

cd ..
python -m http.server 8200 --directory build/html

(If you need to serve this in another port)

Screenshots

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[MaferMazu]

Hello @wlvasquez, I was wondering if you could help me review this documentation PR. Can you? I really appreciate any help you can provide.

[wlvasquez]

A few notes:

1. Replace conjunctions, spell out (you'll for you will)
2. Some actionable steps are in "" better that they be Bold text. Eg. Select Add New.
3. perhaps you may want to create a short bulleted list to accelerate navigation at the beginning of the article. This way people get a quick view of what topics are covered within the article.

[wlvasquez]

Left several comments to help you polish the articles

[MaferMazu]

Thanks for your detailed feedback, @wlvasquez 🙌 I will apply your suggestions.

[MaferMazu]

I already applied your suggestion @wlvasquez. If you want to review only my recent changes, you can visit this: e6642cb
I am attentive if you have more comments or feedback, and thanks again for reviewing this 🙌

[wlvasquez]

Readme.md and readme.rat: lines 40-44 appear to be steps, but are not numbered.

I think you covered everythjng else. Glad I could help 😉

[MaferMazu]

Done @wlvasquez :D
Thanks again for your feedback.

@felipemontoya, can you help me review this?

[felipemontoya]

Great work @MaferMazu. I think this has reached a good state to be merged now.

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@MaferMazu 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.