Document backslash escaping in the url front matter field

[jmooring]

Pending merge/release of gohugoio/hugo#12938.

See gohugoio/hugo#12918 for examples.

Wait until these are resolved:

• resources/page: Escaping colon in permalinks erroneously throws error in some circumstances hugo#12948
• resources/page: Escaping colon in front matter url doesn't work with section pages hugo#12954

Notes:

Hugo does not sanitize the url front matter field. This means you have the ability to generate:

• File paths that contain characters reserved by the operating system. For example, a file path on Windows may not contain a colon (:). Hugo throws an error if you include reserved characters. Learn more about Windows file naming conventions.
• URLs that contain disallowed characters. For example, the less than sign (<) is not allowed in a URL.

If you need to include a colon in the url front matter field, escape it with backslash characters. Use one backslash if you wrap the string within single quotes, or use two backslashes if you wrap the string within double quotes. As described above, this will fail on Windows because the colon (:) is a reserved character.