Upgrade documentation should include mention of clipboard fix for custom themes with custom header file

**Documentation version(s)**:

* 2.5 and later

**Relevant pages**:

* https://www.accesstomemory.org/docs/latest/admin-manual/installation/upgrading

**Relevant sources and links**:

Inspired by the following user forum thread from 2022-07:

* https://groups.google.com/g/ica-atom-users/c/mLG9ry_3yio/m/aCOOuGTLAQAJ

**Description**

* A user with a custom theme, including a customized `_header.php` file, reported the clipboard not working after upgrading from 2.4 to 2.6.4.
* The solution for the user was to add the following line of code to the custom header file:
* https://github.com/artefactual/atom/blob/qa/2.x/apps/qubit/templates/_header.php#L33

```

```

**Recommended solution**

This suggestion should be added to the "[Upgrading with a custom theme](https://www.accesstomemory.org/docs/2.6/admin-manual/installation/upgrading/#upgrading-with-a-custom-theme-plugin)" sections of the 2.6 and 2.7 documentation.

Open the linked “Upgrading with a custom theme” sections in the 2.6 and 2.7 documentation and review the referenced custom header file and forum thread for context. Done means both sections explain the clipboard fix and include the relevant code reference; build the documentation to check the updated pages.

Written by the indexing model from the issue text.