Added jekyll-diagrams error to FAQ (alshedivat#2162)

Signed-off-by: George Araújo <george.gcac@gmail.com>

FAQ.md

@@ -12,6 +12,7 @@ Here are some frequently asked questions. If you have a different question, plea
 - [When I manually run the Lighthouse Badger workflow, it fails with `Error: Input required and not supplied: token`. How do I fix that?](#when-i-manually-run-the-lighthouse-badger-workflow-it-fails-with-error-input-required-and-not-supplied-token-how-do-i-fix-that)
 - [My code runs fine locally, but when I create a commit and submit it, it fails with `prettier code formatter workflow run failed for master branch`. How do I fix that?](#my-code-runs-fine-locally-but-when-i-create-a-commit-and-submit-it-it-fails-with-prettier-code-formatter-workflow-run-failed-for-master-branch-how-do-i-fix-that)
 - [After I update my site with some new content, even a small change, the GitHub action throws an error or displays a warning. What happened?](#after-i-update-my-site-with-some-new-content-even-a-small-change-the-github-action-throws-an-error-or-displays-a-warning-what-happened)
+- [I am trying to deploy my site, but it fails with `Could not find gem 'jekyll-diagrams' in locally installed gems`. How do I fix that?](#i-am-trying-to-deploy-my-site-but-it-fails-with-could-not-find-gem-jekyll-diagrams-in-locally-installed-gems-how-do-i-fix-that)
 
 ---
 
@@ -71,3 +72,7 @@ The `set-output` command is deprecated and will be disabled soon. Please upgrade
 If that's the case, you are using deprecated libraries/commands. This happens because you are using a very old version of al-folio. To fix this it is recommended [upgrading your code to the latest version](INSTALL.md#upgrading-from-a-previous-version) of the template. You will probably need to do some manual merging. If you find it easier, you could create a copy of your repository, do a fresh installation from the template and reapply all your changes. For this I would recommend a tool like [meld](https://meldmerge.org/) or [winmerge](https://winmerge.org/) to check the differences between directories/files.
 
 Note that libraries tend to be deprecated and support for them dropped as they are no longer maintained, and keep using them involves security breaches. Also, some of these deprecations are enforced, for example, by GitHub itself, so there's so much we can do. We have also added tons of new functionality, as well as tidying things up and improving the overall speed and structure, so you could also benefit from these improvements.
+
+#### I am trying to deploy my site, but it fails with `Could not find gem 'jekyll-diagrams' in locally installed gems`. How do I fix that?
+
+`jekyll-diagrams` support was dropped in [#1992](https://github.com/alshedivat/al-folio/pull/1992) in favor of using `mermaid.js` directly. Simply [update your code](INSTALL.md#upgrading-from-a-previous-version) to get the latest changes.