Add Avo::Meta documentation

docs/3.0/meta.md

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+# Meta Properties
+
+Suppose your application has non-developer end users (unthinkable, right?) who want to seamlessly add new properties to a model in a no-code fashion.
+
+The `Avo::Meta` plugin allows just that, and more: It also manages the names and types of these properties (the "meta schema") and does all that without downtime. No database migrations to run, no redeploys, no service disruptions.
+
+Under the hood, it leverages a JSON database column and [StoreModel](https://github.com/DmitryTsepelev/store_model) to manage it.
+
+## Installation
+
+Add the `avo-meta` gem to your bundle:
+
+```ruby
+bundle add "avo-meta"
+```
+
+To use `Avo::Meta`, an additional database table containing the _schemas_ for each resource has to be created. For this, simply install the necessary migrations:
+
+```bash
+$ bin/rails avo_meta:install:migrations
+$ bin/rails db:migrate
+```
+
+## Usage
+
+First, add a `meta` JSON column to your model:
+
+```sh
+$ bin/rails g migration AddMetaToUsers meta:json
+$ bin/rails db:migrate
+```
+
+After that, simply include the `Avo::Metaable` module in said model:
+
+```rb{2}
+class User < ApplicationRecord
+  include Avo::Metaable
+end
+```
+
+This will create two things under the hood:
+
+1. a _"Meta Properties"_ resource available in the sidebar. Here, you can add new meta properties for each associated resource by adding a name and a type. Under the hood, it modifies the `avo_meta_schemas` database table created via the migrations.
+
+This is what this would look like:
+
+<Image src="/assets/img/meta/modify-meta-schema.png" width="1440" height="760" alt="Edit the meta schema of a resource" />
+
+> [!NOTE]
+> If the `meta` JSON column is missing on your model, a reminder to add it will be emitted.
+
+2. a `meta_panel` to be used in your resource `New/Show/Edit` views where you can edit the defined attributes.
+
+To it, simply add `meta_panel` to your `def fields` definition:
+
+```rb{7}
+class Avo::Resources::User < Avo::BaseResource
+  # ...
+
+  def fields
+    # ...
+
+    meta_panel
+  end
+end
+```
+
+<Image src="/assets/img/meta/edit-meta-properties.png" width="1183" height="368" alt="Edit meta properties of a record" />
+
+## Accessing Meta Properties
+
+To use the meta attributes in your application, simply access them like you would access a `has_one` association:
+
+`@user.meta.shoe_size`