Ingest Boostlook 2.0

[julioest]

Description:

We need to figure out a way to temporarily pull in boostlook-v3.css and ingest it into website-v2 so we can render it wherever it's needed.

Source file:
https://github.com/boostorg/boostlook/blob/develop/boostlook-v3.css

Existing references to boostlook.css in website-v2 (places likely needing updates or a parallel path):
https://github.com/search?q=repo%3Aboostorg%2Fwebsite-v2+boostlook&type=code

Goal:
Get boostlook-v3.css loading wherever boostlook.css is currently consumed, in a way that's reversible and doesn't disrupt the existing pipeline.

Open questions / to figure out:

• Pull from the develop branch directly, vendor a copy, or pin to a tag/SHA?
• Load it alongside the current boostlook.css or swap it in?
• Scope: all reference sites at once, or behind a flag / per-page?

[julioest]

For more context, when we're ready for release, we swap back to boostlook.css (with our v3 changes integrated)