Fixed broken link for monthly meetings

[remi-kazeroni]

Description

While doing some reporting, I noticed that the documentation on monthly meetings is outdated since announcements were moved from ESMValTool/issues to Community/Discussions

• Closes #issue_number
• Link to documentation: https://esmvaltool--2806.org.readthedocs.build/en/2806/introduction.html#monthly-meetings

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[bouweandela]

The documentation build is now fixed #2800, could you merge the main branch into this branch so I can review?

[remi-kazeroni]

@bouweandela, this is now ready for review. The changes can be seen in https://esmvaltool--2806.org.readthedocs.build/en/2806/introduction.html#monthly-meetings

[bouweandela]

Thanks!