Create "learn" label so we can keep track of all the learning-related issues in dwyl?

We do a _lot_ of learning in @dwyl projects! (_in fact knowledge is one of the `main` outputs of our work!_)

When someone asks a _question_ in an issue in a project and the issue leads to `new` knowledge,
we are currently only "_OK_" at _capturing_ that knowledge generated in our projects ...

> (_not **everyone** has the **habit** of capturing and sharing knowledge as evidenced by the fact that we have people working with us who have made **zero** contributions to `learn-...` repos/tutorials in the 3 months...! this is **going** to change very soon BTW, but that's for **another issue**!_)

In the interest of being able to _signal_ to other people (_e.g: people who join a project half-way-through_) that we have _learned_ something (_e.g: how to deploy the app on a specific infrastructure_) I _propose_ having a `learn` label we can use in projects!

This will allow people to _filter_ for all the issues where _learning_ was acquired in the project. 💡

I _already_ have this for my _personal_ learning: https://github.com/nelsonic/nelsonic.github.io/issues?q=is%3Aissue+is%3Aopen+label%3Alearn
But I think it's worth having for our projects.

Thoughts?
relates to: https://github.com/dwyl/hq/issues/314