Create last modified date at build time

## Summary

There are at least three use-cases for a last modified date:

* **For google search purposes**: Updates to our sitemap influence when a page is re-indexed.
* **For incremental build purposes**: Updates to a page influence whether or not we should upload new HTML files S3.
* **For LLM indexing purposes**: Updates to a page influence when our LLM indexer re-indexes content.

We may need to calculate and support multiple last-modified dates as what defines a "page change" varies by the use case.

## What determines if a page has changed?

### Sitemap

(to be verified)
Any page changes? Including CSS, Header/Footer, content, and images.

### Incremental Build

(to be verified)
This could be the same as Sitemap? Any page changes? Including CSS, Header/Footer, content, and images.

See https://github.com/elastic/docs-builder/issues/1123 for more information.

### LLM Indexer

Defined in https://github.com/elastic/docs-builder/issues/1146. To summarize:

**Should NOT update last-modified date**
* Configuration changes, like updating CSS, or changing the page layout, image additions/deletions/changes, header and footer changes, navigation changes/reordering.

**Should update last-modified date**
* Content text changes, either to the file being built or to a partial or variable that is included on the page.