Add Builders-substrate-adding built in module article

[augustocollado]

Description

This PR adds a subsection "substrate" in Builders with an article covering how to add a built in module to the frontier template included in tanssi

Checklist

• If this page requires a disclaimer, I have added one
• If pages have been moved around, I have created an additional PR in moonbeam-mkdocs to update redirects

[albertov19]

Looks good. Generally speaking it would be great to a see just a tiny bit of explaining for each code snippet. This feels more like a "follow and don't understand anything you are doing" guide than explaining things (at a high level)

[eshaben]

Mostly just little grammar nitpicks. One thing to note is that all items wrapped in triple back ticks should have a language next to them. For contract addresses, file names, etc. you can use text as the language

There is an index page icon missing for "Adding a Built-in Module" (http://127.0.0.1:8000/builders/build/substrate/)

[albertov19]

Left 1 minor comment but otherwise looks good.

Create a ticket to restructure this content once we have the new flow of everything being based of Tanssi templates with the new Prerequisites that Devs need to have a Tanssi template running or something

[albertov19]

Looks great, left some minor feedback, feel free to include it and merge it 🙌

[themacexpert]

Approved