Add a warning at the top of all Pipelines pages in the Legacy (v1) section about v2

[hbelmiro]

We should place a warning at the top of all pages in the Legacy (v1) section that says something like:

This page is for Kubeflow Pipelines V1.
While the V2 backend is [backwards compatible](xxx_v1_v2_compatibility_xxxx) with V1 pipelines, 
we recommend you [upgrade to the V2 SDK](xxxx_v2_migration_guide_xxxx), please see the [V2 docs](xxxx_v2_docs_xxxx) for more information.

The xxx_v1_v2_compatibility_xxxx page can probably be the References / Version Compatibility page, but it needs to be made clearer and brought up to date for the 2.1 and 2.2 releases.

Originally from: #3712 (comment)

/area pipelines

[hbelmiro]

Duplicated by #3751.