Docs: added an accessibility section in the pull request guide (#36454)

* Docs: added an accessibility section in the pull request guide

* added more points to the accessibility guideline

* fixes some grammar nits

* changed aria to capital case since its an acronym

* added a reference to the Grafana theme

* improved the wordings for better context

contribute/create-pull-request.md

@@ -48,6 +48,13 @@ Pull requests that add or modify unit tests that are written in Jest must adhere
 - Don't add snapshots tests. We are incrementally removing existing snapshot tests, we don't want more.
 - If an existing unit test is written in Enzyme, migrate it to RTL (React Testing Library), unless you’re fixing a bug. Bug fixes usually shouldn't include any bigger refactoring, so it’s ok to skip migrating the test to RTL.
 
+Pull requests that create new UI components or modify existing ones must adhere to the following accessibility guidelines:
+
+- Use semantic HTML.
+- Use ARIA roles, labels and other accessibility attributes correctly. Accessibility attributes should only be used when semantic HTML doesn't satisfy your use case.
+- Use the [Grafana theme palette](/contribute/style-guides/themes.md) for styling. It contains colors with good contrast which aids accessibility.
+- Use [RTL](https://testing-library.com/docs/dom-testing-library/api-accessibility/) for writing unit tests. It helps to create accessible components.
+
 ### Backend-specific guidelines
 
 Please refer to the [backend style guidelines](/contribute/style-guides/backend.md).
@@ -107,6 +114,6 @@ Make sure that the title for your pull request uses the same format as the subje
 
 If your PR includes configuration changes, all of the following files must be changed correspondingly:
 
-* conf/defaults.ini
-* conf/sample.ini
-* docs/sources/administration/configuration.md 
+- conf/defaults.ini
+- conf/sample.ini
+- docs/sources/administration/configuration.md