Improves documentation regarding providers and custom connections

[soltanianalytics]

I had some issues figuring out how to make custom connections work. Thankfully, @potiuk was very supportive in the airflow slack channel and helped me out. I'm happy to contribute to the docs in a way that I believe I would have found very helpful initially.

In particular:

• The How to create your own provider section should be on the main level, otherwise it is easy to overlook and also is confusing with the very similarly named Creating your own providers subsection of the FAQ for Airflow and Providers - especially since you may be linked to this page from the connection page, looking for information on, well, creating your own providers
• I added a verbose section on what exactly to do to make a package a provider
• I took the liberty to also make some minor language improvements wherever I stumbled across an opportunity to do so

Please feel free to use it as-is, make any additional changes, or ignore my suggestions altogether.

Big thanks again to @potiuk for helping me out! The custom connections feature in particular is pretty cool!

[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/master/CONTRIBUTING.rst)
Here are some useful points:

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[potiuk]

❤️

[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 master or amend the last commit of the PR, and push it with --force-with-lease.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!