api-docs: Move markdown files to top level directory.

- Updates `.prettierignore` for the new directory.
- Updates any reference to the API documentation directory for
  markdown files to be `api_docs/` instead of `zerver/api/`.
- Removes a reference link from `docs/documentation/api.md` that
  hasn't referenced anything in the text since commit 0542c60.
- Update rendering of API documentation for new directory.

api_docs/api-doc-template.md

@@ -0,0 +1,31 @@
+{generate_api_header(API_ENDPOINT_NAME)}
+
+## Usage examples
+
+{start_tabs}
+
+{generate_code_example(python)|API_ENDPOINT_NAME|example}
+
+{generate_code_example(javascript)|API_ENDPOINT_NAME|example}
+
+{tab|curl}
+
+{generate_code_example(curl)|API_ENDPOINT_NAME|example}
+
+{end_tabs}
+
+## Parameters
+
+{generate_api_arguments_table|zulip.yaml|API_ENDPOINT_NAME}
+
+{generate_parameter_description(API_ENDPOINT_NAME)}
+
+## Response
+
+{generate_return_values_table|zulip.yaml|API_ENDPOINT_NAME}
+
+{generate_response_description(API_ENDPOINT_NAME)}
+
+#### Example response(s)
+
+{generate_code_example|API_ENDPOINT_NAME|fixture}