Improvements on event observer topic

[ajithkumar-maragathavel]

Purpose of this pull request

This pull request (PR) includes the below improvements:

• Added example of subscribing to the custom event.
• Added information on the list of valid areas for observers.

Affected DevDocs pages

• https://devdocs.magento.com/guides/v2.3/extension-dev-guide/events-and-observers.html
• https://devdocs.magento.com/guides/v2.4/extension-dev-guide/events-and-observers.html

Links to Magento source code

• https://github.com/magento/magento2/blob/2.4-develop/app/code/Magento/Sales/etc/events.xml

whatsnew
Added an example and table to the Events and observers.

[BarnyShergold]

Good addition

[ajithkumar-maragathavel]

@BarnyShergold Updated the PR with the suggested changes. Thanks

[keharper]

@ajithkumar-maragathavel A few editorial comments. The 3rd column of the table isn't necessary since 6 out 7 descriptions vary by one word.

[ajithkumar-maragathavel]

@keharper Thanks for the suggestions. PR updated.

[keharper]

@magento import code to magento-devdocs/devdocs

[magento-engcom-team]

@keharper the branch with code successfully imported intomagento-devdocs/devdocs repository. Branch name: imported-magento-devdocs-8950.

[ghost]

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