Remove the 3rd-party references from PMC "apache.org" site

[potiuk]

There are still a couple of things that are linked to / hosted to dagworks: blog, youtube video, link to Dataflow Hub. They are a bit scattered across the site (similar but a bit less extensive in burr). There are "External Resources" but also some links are from inside the docs in various pages.

The approach we took in Airflow, to keep to the vendor-neutrality spirit there, is that we made sure from the very beginning that all the links out to stakeholders (and basically any other external 3rd-parties interacting with Airflow) are placed here:

https://airflow.apache.org/ecosystem/

This helps people when they look for resources relevant to Airflow to understand what is controlled and endorsed by PMC and what is not - we made sure that this header is there:

These resources and services are not maintained, nor endorsed by the Apache Airflow® Community and Apache Airflow project (maintained by the Committers and the Airflow PMC). Use them at your sole discretion. The community does not verify the licences nor validity of those tools, so it’s your responsibility to verify them.

And we took the approach that whoever wants to add a relevant resource there - they can do it and we will not make "decisions" about what goes in other than a quick review to see if it is indeed "relevant". This might be something you might be considering doing at some point of time. I think it is important to separate what the PMC does from what "stakeholders" do - carefully avoiding that official PMC communication ("hamilton.apache.org" is such a communication) does not endorse or suggests any 3rd-parties as more important than others and something that the PMC depends on.

Decisions on what to do and how to approach it should be made - likely by PPMC before it is executed.