Contributor Docs: add provider documentation standards page

contributing/topics/documentation-standards.md

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+# Provider Documentation Standards
+
+In an effort to keep the [provider documentation](https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs) consistent, this page documents some standards that have been agreed on. 
+
+This page will grow over time, and suggestions are welcome!
+
+## Notes
+
+Note blocks are used to provide additional information to users beyond the basic description of a resource, argument or attribute.
+
+In the past, there have been different approaches to how notes were formatted, some examples are:
+- Different words to indicate level of importance, e.g. `Info`, `Important`, `Caution`, and `Be Aware`.
+- Capitalisation differences, e.g. `Note:` vs `NOTE:`.
+- Whether or not a colon is included, e.g. `Note:` vs `Note`.
+
+Going forward, all notes should follow the exact same format (`(->|~>|!>) **Note:**`) where level of importance is indicated through the different types of notes as documented below. 
+
+<!-- 
+    - TODO: Considerations for when to add notes? We probably don't want to overdo it (More relevant to informational notes)
+-->
+
+### Informational Note
+
+Informational note blocks should generally be used when a note provides additional useful information, recommendations and/or tips to the user.
+
+To add an informational note, use `-> **Note:**`, within the Terraform registry documentation this will template as a block with an info icon.
+
+For example, extra information on the supported values for an argument, possibly linking to external documentation for the resource/service:
+
+```markdown
+* `type` - (Required) The type. Possible values include `This`, `That`, and `Other`.
+
+-> **Note:** More information on each of the supported types can be found in [type documentation](link-to-additional-info)
+```
+
+### Warning Note
+
+Warning note blocks should generally be used when a note provides information that the user will need to avoid certain errors, however if these errors are encountered they should not break anything or cause irreversible changes.
+
+To add a warning note, use `~> **Note:**`, within the Terraform registry documentation this will template as a block with a warning icon.
+
+For example, an argument that is optional but required when another argument is set to `true`:
+
+```markdown
+* `optional_argument_enabled` - (Optional) Is the optional argument enabled? Defaults to `false`.
+
+* `optional_argument` - (Optional) An optional argument.
+
+~> **Note:** The argument `optional_argument` is required when `optional_argument_enabled` is set to `true`.
+```
+
+### Caution Note
+
+Caution note blocks should generally be used when a note provides critical information on potential irreversible changes, data loss or other things that can negatively affect a user's environment.
+
+To add a caution note, use `!> **Note:**`, within the Terraform registry documentation this will template as a block with a caution icon.
+
+For example, an argument that when set to `true` cannot be reversed without recreating the resource:
+
+```markdown
+* `irreversible_argument_enabled` - (Optional) Is irreversible argument enabled? Defaults to `false`.
+
+!> **Note:** The argument `irreversible_argument_enabled` cannot be disabled after being enabled.
+```