Backport ShowUnroutableContentWarnings to V13

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

#17837 introduced a configuration option (ShowUnroutableContentWarnings) to disable warnings for unroutable content in V15 (the warnings were originally introduced for V13 in #17705).

This PR backports the same configuration from V15 to V13 😄

Testing this PR

1. Create content with conflicting routes.
2. Verify that warnings are conditionally displayed at publish time for that content, depending on the current value of ShowUnroutableContentWarnings

Note that you must have ShowDomainWarnings disabled to reliably test this, because ShowDomainWarnings overrules ShowUnroutableContentWarnings in certain scenarios:

"Umbraco": {
    "CMS": {
      "Content": {
        "ShowDomainWarnings": false,
        "ShowUnroutableContentWarnings": true/false
      }

[AndyButland]

I've tested this by creating two content items with the same route using the umbracoUrlName magic property. Can confirm the warning only appears when configured to do so.

Just to note, we've missed documenting this for 15, but I've included now in this PR for 15.3: umbraco/UmbracoDocs#6879

As well as this one for 13.8: umbraco/UmbracoDocs#6876