Add docs for explicit-heading

docs/rules/explicit-heading.md

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+## Require explicit heading level on `<Heading>` component
+
+The `Heading` component does not require you to use `as` to specify the heading level, as it will default to an `h2` if one isn't specified. This may lead to inaccessible usage if the default is out of order in the existing heading hierarchy. 
+
+## Rule Details
+
+This rule enforces using `as` on the `<Heading>` component to specify a heading level (`h1`-`h6`). In addition, it enforces `as` usage to only be used for headings.
+
+👎 Examples of **incorrect** code for this rule
+
+```jsx
+import {Heading} from '@primer/react'
+
+<Heading>Heading without explicit heading level</Heading>
+```
+
+`as` must only be for headings (`h1`-`h6`)
+
+```jsx
+import {Heading} from '@primer/react'
+
+<Heading as="span">Heading component used as "span"</Heading>
+```
+
+👍 Examples of **correct** code for this rule:
+
+```jsx
+import {Heading} from '@primer/react';
+
+<Heading as="h2">Heading level 2</Heading>
+```
+
+## Options
+
+- `skipImportCheck` (default: `false`)
+
+  By default, the `explicit-heading` rule will only check for `<Heading>` components imported directly from `@primer/react`. You can disable this behavior by setting `skipImportCheck` to `true`.