docs: add the standard README header to multimodel and incremental

[em-anyscale]

Adds the standard header to the multimodel and incremental READMEs: the "Run on Anyscale / View on GitHub" badge row and a "Time to complete" line. Both were missing it in the console preview.

• Incremental: about 12 min, including cluster start, from its measured CI run (11 min 41 s).
• Multimodel: about 7 min without a Hugging Face token, from the ungated test run. The gated run has not been timed end to end, so the line says it takes longer without giving a number.

README.md is regenerated from README.ipynb with nbconvert; only the seven header lines change in each.