Ignore autosummary warning in documentation build

[bouweandela]

Description

I'm not sure if this is the right way to solve this issue, but at least it fixes the build. If anyone has better ideas, that would be most welcome.

Successful build: https://app.circleci.com/pipelines/github/ESMValGroup/ESMValCore/11389/workflows/7ef9bd62-5a41-4aa6-954f-cf7e031876ec/jobs/47859

Closes #2479

Link to documentation: https://esmvaltool--2480.org.readthedocs.build/projects/ESMValCore/en/2480/

---

Before you get started

• [x] ☝ Create an issue to discuss what you are going to do

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• [x] 🧪 The new functionality is relevant and scientifically sound
• [x] 🛠 This pull request has a descriptive title and labels
• [x] 🛠 Code is written according to the code quality guidelines
• [x] 🧪 and 🛠 Documentation is available
• [x] 🛠 Changes are backward compatible
• [x] 🛠 The list of authors is up to date
• [x] 🛠 All checks below this pull request were successful

---

To help with the number pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.64%. Comparing base (268dee4) to head (b627777).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2480 +/- ## ======================================= Coverage 94.64% 94.64% ======================================= Files 249 249 Lines 14114 14114 ======================================= Hits 13358 13358 Misses 756 756 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[valeriupredoi]

I actually asked a question on that Sphinx issue. the solution recommended is a bit of a workaround, and not a correct fix :+1: