Move Backport Providers docs to our docsite

[kaxil]

The Backport Providers guide is more suitable for docsite. I have improved grammatical errors wherever necessary to while moving it. This also makes our README.md a bit lighter.

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Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
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In case of backwards incompatible changes please leave a note in UPDATING.md.

[potiuk]

Nice. I guess we shoudl also cherry-pick it to the v1-10-test branch for 1.10.13. i can do it.

[mik-laj]

It seems to me that we have several products and each product has its own documentation. We therefore should update the index of our documentation - https://airflow.apache.org/docs/ to show this, so that we can easily update any documentation independently.
I imagine there will be a list of documentation links for each of our products, such as Apache Airflow, Helm Charm for Apache Airflow, Docker Image, Backport packages and others.
Somethings similar to: https://mc-stan.org/users/documentation/ https://cloud.google.com/sql/docs

[potiuk]

@mik-laj -> maybe create an issue for it? That's an interesting proposal that seems like a major doc restructuring - impacting all the stages - updating, building, releasing. And one that might finally speed up the documentation building process. Sounds like an interesting candidate for 2.1 :)

[mik-laj]

@potiuk will do.

[kaxil]

Yea I like the proposal @mik-laj too

[mik-laj]

I created an issue: #11152