Remove sidebar: subtitle (#1530)

Doesn't appear to exist: https://github.com/quarto-dev/quarto-cli/blob/d0d6f87e18e7522386bc8b1ab5b390ef7eca3799/src/resources/schema/definitions.yml#L997-L1083

Author: cwickham

docs/websites/website-navigation.qmd

@@ -164,7 +164,6 @@ Here are all of the options available for side navigation:
 |------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | `id`             | Optional identifier (used only for hybrid navigation, described below).                                                                                                                                                                                            |
 | `title`          | Sidebar title (uses the project title if none is specified).                                                                                                                                                                                                       |
-| `subtitle`       | Optional subtitle.                                                                                                                                                                                                                                                 |
 | `logo`           | Optional logo image.                                                                                                                                                                                                                                               |
 | `logo-alt`       | Alternate text for the logo image.                                                                                                                                                                                                                                 |
 | `logo-href`      | Target href from sidebar logo. By default, the logo and title link to the root page of the site (`/index.html`).                                                                                                                                                   |