Turn task-sdk into peer-citizen same as other doc packages

[potiuk]

The task-sdk package did not have full integration with airflow docs:

• did not have inventory mapping
• did not have a way to refer to it via doc:task-sdk: directive 8 did not have a development link from the devel index

This PR fixes all that.

---

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In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.

[potiuk]

One of the things this one enables is capability to refer to task-sdk docs same as all other packages - using intersphinx package reference and inventories.

This way staging for example will work - currently the link on staging has hard-coded link to a missing "live" task-sdk, where with the intersphinx reference, we can refer to "airflow's" task-sdk package and the links will be properly generated - and also verified if they exist - as relative references rather than hard-coded absolute links:

[github-actions]

Backport failed to create: v3-0-test. View the failure log Run details

Status	Branch	Result
❌	v3-0-test

You can attempt to backport this manually by running:

cherry_picker 244af72 v3-0-test

This should apply the commit to the v3-0-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue