chore: naming custom domain (#11383)

# Which Problems Are Solved

As part of the consistent naming effort, this PR focuses on "Custom
Domains".

# How the Problems Are Solved

- All terms referring to a domain identifying an instance were changed
to "Custom Domain" or "custom domain".
- All placeholders were changed to `${CUSTOM_DOMAIN}` or
`$CUSTOM_DOMAIN` (if escaping was not possible)
- Some other placeholders were change to the same style.
- One occurrence of custom domain was changed to organization domain to
prevent misunderstandings.

# Additional Changes

None

# Additional Context

- closes #11296 
- customer portal is fixed on
https://github.com/zitadel/website/pull/1570
- angular example is updated on
https://github.com/zitadel/zitadel-angular/pull/29

docs/src/components/environment.jsx

@@ -39,7 +39,7 @@ export function SetEnvironment() {
   return (
     <div>
       <div className={styles.inputwrapper}>
-        <label className={styles.label}>Your custom domain</label>
+        <label className={styles.label}>Your Custom Domain</label>
         <input
           className={styles.input}
           id="instance"