chore(docs): remove SPDX header from component docs

[ShGKme]

☑️ Resolves

• Fix License info in online docs #5808

When there is nothing else in a mixin, styleguidist uses SPDX header as mixin's JSDoc and adds it to the component docs.

/**
 * SPDX-FileCopyrightText: 2020 Nextcloud GmbH and Nextcloud contributors
 * SPDX-License-Identifier: AGPL-3.0-or-later
 */

export default {
  // Mixin

Affected components:

• NcHeaderMenu
• NcAppNavigationSettings
• NcActionCheckbox
• NcActionInput
• NcActionLink
• NcActionRouter
• NcActionRadio
• NcActionText
• NcActionTextEditable

This can be disabled via validExtends config, but that would also remove props docs defined in mixins.

A solution - add a fake empty JSDoc in a mixin.

Alternative solution

Get rid of mixins

🖼️ Screenshots

🏚️ Before	🏡 After

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 3️⃣ Backport to next requested with a Vue 3 upgrade

[ChristophWurst]

Nice

[ShGKme]

Force pushed a typo fix: bellow -> below 🐮