Add documentation for logic customization

[sinaeftekhar]

🎩 What? Why?

Currently, decidim documentation is lacking logic customization. This section is added to the "Getting started section.

📋 Checklist

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[ahukkanen]

Overall this looks great but I think it needs some refining. I've left some suggestions below.

Also, I noticed we already have a "Customize" section in the documentation, e.g. how to customize the views:
https://docs.decidim.org/en/customize/views/

Or at GitHub:
https://github.com/decidim/decidim/tree/develop/docs/modules/customize/pages

I think this would be a natural place for this piece of documentation, we can name it e.g. customize/logic, so it would become available at this URL after these changes:
https://docs.decidim.org/en/customize/logic/

As we discussed in another issue, the documentation might now be perfectly structured right now, but for this particular addition, let's first add it where it naturally belongs and later fix the issues with the navigational structure as a whole.

[sinaeftekhar]

@ahukkanen
All review issues addressed.

[andreslucena]

Great addition and contribution @sinaeftekhar!
I have some feedback, can you give it a look please? Thanks

[ahukkanen]

Few more improvement suggestions + please review one of the previous comments (I marked it as unresolved above).

[ahukkanen]

@andreslucena This is good for me, so feel free to merge once you are happy with it.

Regarding your previous comment.

It feels like the idea of this page is really similar to https://github.com/decidim/decidim/blob/develop/docs/modules/customize/pages/code.adoc, but with much more detail. So should we delete that and use this? Maybe adding the real world examples and also the modules part?

We have gone through several iterations of doing customizations within an instance and figured the concern way is probably the cleanest way to achieve this and keep track of what has changed.

The decorator approach in the linked example doesn't look too bad but we prefer the concern way.

As a real life example, I think the "Simple Proposal" module we've built is quite a good real life example (we use it with almost all of the smaller clients):
https://github.com/mainio/decidim-module-simple_proposal

Of course id doesn't demonstrate how to do the customizations directly from the application code but there's nothing stopping you from applying similar customizations from the application itself.

[andreslucena]

figured the concern way is probably the cleanest way to achieve this and keep track of what has changed

To me too, of course.

I was only talking about having two similar but different sections in the documentation navigation for modifying Ruby code. But I actually don't have it clear on how to do it, so when I have a proposal I'll make the PR.

For sure, this is a clear improvement on what we have now!!

[andreslucena]

LGTM, thanks!