URL in section title breaks automatic link

We noticed that a section heading `### The netplan.io approach` was rendered incorrectly on the docs website: there was an automatic link for the section heading + an automatic link for the `netplan.io` URL-like part.

We fixed it by adding a backslash: `### The netplan\.io approach`, but this seems like a bug that needs to be addressed.

The affected page is: [How to create an instance](https://multipass.run/docs/create-an-instance?fff)

Open the affected How to create an instance page and reproduce the issue with the heading `### The netplan.io approach`, comparing it with the escaped version. Trace the documentation heading and automatic-link rendering path, then verify that the heading produces only its section link without a second link around `netplan.io`.

Written by the indexing model from the issue text.