[Broken Link] Fix broken links in dot-inventory.md

## What needs to change
The Department of Transportation data inventory case study contains more than 10 broken links. Most are old DOT URLs that have moved or been restructured. This case study still has value as a historical example so it should be updated rather than deleted, but links that cannot be found should be removed.

## Where to find it
`pages/_resources/dot-inventory.md`

## Broken links to fix
Work through each one — for each URL try replacing `http://` with `https://` first as some may simply need the protocol updated:

- `http://www.dot.gov/open/data` — try https://www.transportation.gov/data
- `http://www.dot.gov/data` — try https://www.transportation.gov/data
- `https://www.transportation.gov/tags/open-government` — check if this tag page still exists
- `http://www-odi.nhtsa.dot.gov/downloads/` — try https://www.nhtsa.gov/research-data
- `http://ai.fmcsa.dot.gov/` — check if FMCSA AI resources moved to fmcsa.dot.gov
- `https://www.fmcsa.dot.gov/safety/research-and-analysis/fmcsa%E2%80%99s-mobile-developer-website-and-web-services` — remove this link if no equivalent exists
- `https://www.bts.gov/` — try the link directly, BTS may be blocking the checker but the page may still exist
- `https://www.transportation.gov/sites/dot.gov/files/docs/OMB%20M-10-06.pdf` — search for this memo on govinfo.gov instead

For any link that cannot be resolved after checking, remove it from the page rather than leaving a dead link.

## How to verify
After updates run:
```
lychee --base=https://resources.data.gov pages/_resources/dot-inventory.md
```
Confirm zero errors.