Add "disappearing DAGs" section on FAQ doc

[rtrindvg]

Adds section about disappearing DAGs on UI, as discussed on #44495.

I tried to make the new question have the same tone as the rest of the FAQ, so I condensed A LOT of the explanations in shorter descriptions and used links to point to more complete explanations, as directed by @potiuk in the original discussion. If I went to far in this direction, I can try to increment it.

Also I am not proficient with Sphinx internal links syntax, but I tried my best to make sure they are correct.

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^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide (https://github.com/apache/airflow/blob/main/contributing-docs/README.rst)
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[potiuk]

Nice!

[potiuk]

You can folow the https://github.com/apache/airflow/blob/main/contributing-docs/11_documentation_building.rst to build the docs locally and check if links render correctly

[potiuk]

Some static checks need fixing :)

[potiuk]

Best - install pre-commit and it will fix them automatically for you

[phanikumv]

@rtrindvg just checking in to see if you need help to resolve the CI failures here

cc @amoghrajesh

[amoghrajesh]

@rtrindvg yeah it should be a simple fix, let me still know if you need help

[amoghrajesh]

@rtrindvg i just pushed a fix to your branch to fix static checks. FYI @phanikumv

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.

[github-actions]

Backport successfully created: v3-0-test

Status	Branch	Result
✅	v3-0-test

[rtrindvg]

@rtrindvg i just pushed a fix to your branch to fix static checks. FYI @phanikumv

Thank you so much, just really swamped with a career turmoil at the moment and could not spend time on this for now. Thanks for the fix and the subsequent merge.