docs: how to use layers

[IronCore864]

Docs: How to use layers.

As previously discussed, the layers reference doc is removed and refactored into a how-to.

Preview: https://canonical-pebble--548.com.readthedocs.build/en/548/how-to/use-layers/

[dwilding]

This should be 003-env-override-test.yaml shouldn't it?

[IronCore864]

The idea is that override-test will only be put into the test env and override-dev will only be put in the dev env. So it doesn't matter if they have same numeric prefixes because they will not be put into the same env.

[dwilding]

003-env-override-test.yaml ?

[dwilding]

Thanks @IronCore864, this new doc is a helpful summary of how to use layers! I've added some suggestions and questions about specific parts.

One other observation - In the intro, the doc mentions three benefits:

1. organize services into logical groups
2. define one set of configurations per environment
3. layered configurations can also accommodate imperative overrides

Of these, it seems that we only illustrate 1 and 3. Is that because 2 is pretty similar to 1 in approach?

I think it would be nice if we could match up each benefit with a corresponding section. For example, at the end of the first bullet, we could say:

For an example, see Use layers as logical groups.

And similarly for the other benefits.

We could either add a dedicated section to show how to define one set of configurations per environment. Or if it's really essentially the same as how to organize services into logical groups, at the end of benefit 2 we could say something like:

The way to achieve this in Pebble is the same as how to organize services into logical groups, with each environment set up as a separate group.

What do you think?