docs(guidelines): add namespacing guidelines for DXP

guidelines/dxp/namespacing.md

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+# Namespacing guidelines
+
+To promote consistency, we have established the following rule of thumb for namespacing in DXP:
+
+-   Unique attributes, such as `id`s, should _not_ be namespaced
+-   Non-unique attributes, such as `name`, should be namespaced
+-   Fallback: if `id` is not passed, default to using `name` as `id`
+
+## Reasoning
+
+The purpose of the `id` attribute is to define something unique to the whole page. By following this approach, we ensure that these attributes can be used in methods such as `document.getElementById` as is, without juggling namespaces.
+
+Other attributes, such as `name`, only need to be unique for some internal mechanism, such as in a `form` element. By namespacing them internally we ensure that, in case they end up being used under the hood as `id`s, they won't bleed into other components and cause duplicate issues.
+
+## Examples
+
+### Unique attributes
+
+Take the following snippet:
+
+```html
+<aui:form id="fm" namespace="myNamespace_">
+	<clay:headless-data-set-display formId="fm" namespace="myNamespace_" />
+</aui:form>
+```
+
+The `clay:headless-data-set-display` element receives the `id` of its wrapping form as a `formId` attribute. This `id` **doesn't include the namespace**, and should not be namespaced inside the form taglib. The `clay:headless-data-set-display` component can now internally use `document.getElementById(formId)` to interact with the form when necessary, without adding the `namespace` before it, because the resulting HTML for the `form` would be as follows:
+
+```html
+<form id="fm">...</form>
+```
+
+### Non-unique attributes
+
+Take this other example:
+
+```html
+<aui:form name="fm" namespace="myNamespace_">
+	<clay:headless-data-set-display formName="fm" namespace="myNamespace_" />
+</aui:form>
+```
+
+In this case, the `form` tag has a `name` attribute, which **will be namespaced under the hood**. Since we aren't passing an `id` prop, the component must use the fallback case and use the `name` as an `id`, resulting in the following HTML:
+
+```html
+<form id="myNamespace_fm" name="myNamespace_fm">...</form>
+```
+
+In this case, for the `clay:headless-data-set-display`, we are passing the `formName` prop instead of `id`. The component must namespace the `formName` attribute to find the element on the page, as such:
+
+```js
+const formElement = document.getElementById(`${namespace}${formName}`);
+```