add warning about pages for repositories with a dot in the name (#670)

Hi,

coming from https://codeberg.org/Codeberg/pages-server/issues/512, this adds a warning that repos with dots in the name can cause issues when trying to set up a custom domain.

I'm not sure if the wording here fits the existing docs, or if hints like this fit in the docs at all, but I wanted to open this as a suggestion nonetheless :)

Reviewed-on: https://codeberg.org/Codeberg/Documentation/pulls/670
Reviewed-by: Bastian Greshake Tzovaras <gedankenstuecke@noreply.codeberg.org>
Co-authored-by: Kilian Koeltzsch <me@kilian.io>
Co-committed-by: Kilian Koeltzsch <me@kilian.io>

content/codeberg-pages/using-custom-domain.md

@@ -83,6 +83,20 @@ Depending on from where you want to serve your files, there is a naming scheme f
  </tbody>
 <table>
 
+{% admonition "warning" "Repository names with dots are not supported" %}
+
+Repository names containing dots (`.`) are **not supported** for custom domains. The Pages server uses dots as delimiters to parse the repository name, branch, and owner from DNS records.
+
+For example, if your repository is named `my.project`, the TXT record `my.project.username.codeberg.page` will be incorrectly parsed as:
+
+- Branch: `my`
+- Repository: `project`
+- Owner: `username`
+
+**Solution:** If you need to use a custom domain, rename your repository to use hyphens (`-`) or underscores (`_`) instead of dots. For example, rename `my.project` to `my-project`.
+
+{% endadmonition %}
+
 _We assume for the following description, that you want to serve your website with and without www in front.
 If you are on a subdomain already (like myproject.yourdomain.com), you can skip the `www.yourdomain.com` CNAME entry._