Add datapipes deprecation warning banner to docs (#1312)

[andrewkho]

Cherry pick docs banner change

cd docs
make html
open build/html/index.html

screenshot:

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/data/1313

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 2 New Failures

As of commit 131349b with merge base f827b0d ():

NEW FAILURES - The following jobs have failed:

• Run AIStore Datapipe Test / test (macos-latest, 3.8) (gh)
  Process completed with exit code 2.
• Run AIStore Datapipe Test / test (ubuntu-latest, 3.8) (gh)
  Process completed with exit code 2.

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[gokulavasan]

lg!