Webpage Integration: Library Subpage Hero Illustrations

_Draft content moved from https://github.com/boostorg/website-v2/issues/2430_

----

### Page variants

The library subpage supports two display variants based on a library's flagship status.

**Flagship libraries**

- A library is identified as flagship via a `flagship` tag in the database.
- Flagship libraries display a unique illustration in the page header.
- A mechanism must be built to associate a specific illustration asset with each flagship library. Illustrations are stored in S3 — the library record in the database should store a reference to the S3 URL of its illustration.
- All flagship libraries must have an illustration, but in the rare case of no associated illustration, use the fallback placeholder.
- PM Note: Some flagship libraries may not have illustrations. In this case, use default hero section
`jlchilders11` Dev Note: to facilitate this, the illustration field should be nullable, and the updated hero component should account for this by loading the empty state version.

**Standard libraries**

- All libraries without the `flagship` tag render the standard subpage layout without an illustration.

Start by locating the library subpage and its hero component, then inspect the library database record and how S3 asset URLs are represented. Implement the flagship and standard page variants, including the nullable illustration fallback. Done means flagship libraries can load their associated illustrations or the empty state, while standard libraries show no illustration.

Written by the indexing model from the issue text.