URLs in curl commands should be quoted

Attempts to create a Vale rule for identifying curl commands with
URLs that are not escaped in quotation marks.

doc/.vale/gitlab/CurlStringsQuoted.yml

+---
+# Error: gitlab.CurlStringsQuoted
+#
+# Ensures all codeblocks using curl quote any URL strings.
+#
+# For a list of all options, see https://errata-ai.github.io/vale/styles/
+extends: existence
+message: 'Curl commands must wrap URLs in double quotes ("): %s'
+link: https://docs.gitlab.com/ee/development/documentation/styleguide.html#code-blocks
+level: warning
+scope: code
+raw:
+  - 'curl.*[^"=]https?://.*'

doc/development/documentation/styleguide.md

@@ -1614,7 +1614,7 @@ Rendered example:
 - Wherever needed use this personal access token: `<your_access_token>`.
 - Always put the request first. `GET` is the default so you don't have to
   include it.
-- Use double quotes to the URL when it includes additional parameters.
+- Wrap the URL in double quotes (`"`).
 - Prefer to use examples using the personal access token and don't pass data of
   username and password.