Adds Pendulum 1.x -> 2.x upgrade documentation

[1nF0rmed]

closes: #18634

Adds documentation about the upgrade from Pendulum 1.x to 2.x as discussed in the issue.

Assumptions that were made:

• Most of the Pendulum changes are already documented in the official Pendulum docs.

Added the following:

• Mention the upgrade from 1.x to 2.x
• Added an example of a code snippet that will now throw errors
• Added link to official pendulum 2.x docs that discuss the changes from 1.x to 2.x

The macros documentation as mentioned in the issue were actually pointing to the updated Pendulum documentation, so no changes were added for the same. For instance, consider the link for the macro prev_execution_date

As it was a documentation update, didn't run any tests.

[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 Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
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[github-actions]

The PR is likely ready to be merged. No tests are needed as no important environment files, nor python files were modified by it. However, committers might decide that full test matrix is needed and add the 'full tests needed' label. Then you should rebase it to the latest main or amend the last commit of the PR, and push it with --force-with-lease.

[kaxil]

Static check is failing

[boring-cyborg]

Awesome work, congrats on your first merged pull request!