[Content Gap] Update Statistical Policy Directive No. 15 cross-references across the site

## What needs to change
Issue 3 in M1 updates the primary SPD-15 listing to the 2024 revision. However SPD-15 may be referenced or cross-linked in other pages across the site. Those references also need to be checked and updated so no page is still pointing visitors toward the superseded 1997 version.

## What to do
Search the repo for all references to SPD-15 or the race and ethnicity standards:
```
grep -r "SPD-15\|Directive.*15\|race.*ethnicity.*standards\|1997-10-30\|97-28653" pages/
```

For each result check whether the reference links to or describes the 1997 version. If so update it to reference the 2024 revision using the URL and description established in Issue 3.

## Also check
- The Data Standards section at https://resources.data.gov/standards — confirm SPD-15 appears correctly if listed there
- Any data standards YAML or JSON data files in the repo that may contain SPD-15 metadata. Search:
```
grep -r "SPD-15\|race.*ethnicity" pages/_data/
```

## How to verify
Search the live site at https://resources.data.gov for "Statistical Policy Directive 15" and confirm all results reference the 2024 revision, not the 1997 version.