Default PDF build does not support sphinx.tabs

The default `.readthedocs.yaml` file includes `pdf` as one of the output formats (aside from HTML).

The generated PDF does not support tabbed content. The tab title + content is printed properly for the last tab; but only the tab titles are printed for the rest.

Screenshot of PDF output:

Example PDF with tabbed content:
[canonical-kernel-docs-readthedocs-hosted-com-en-latest.pdf](https://github.com/user-attachments/files/17640452/canonical-kernel-docs-readthedocs-hosted-com-en-latest.pdf)

Tabbed content source:
~~~
`````{tabs}
````{tab} Noble Numbat 24.04 (and newer)

Add "deb-src" to the `Types`: line in the
{file}`/etc/apt/sources.list.d/ubuntu.sources` file.

```{code-block} text
:emphasize-lines: 1

Types: deb deb-src
URIs: http://archive.ubuntu.com/ubuntu
Suites: noble noble-updates noble-backports
Components: main universe restricted multiverse
Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg
```
````

````{tab} Mantic Minotaur 23.10 (and older)

Check that you have the following entries in the {file}`/etc/apt/sources.list`
file. If not, add or uncomment these lines for your Ubuntu release.
```{code-block} text

deb-src http://archive.ubuntu.com/ubuntu jammy main
deb-src http://archive.ubuntu.com/ubuntu jammy-updates main
```
````
`````
~~~