Improve the UX of failed vulnerability retrieval by the API

**Describe the bug**
Users try to retrieve a vulnerability (typically a CVE) by the API that the web interface reports as an alias of another vulnerability and that retrieval fails because the CVE record does not exist in OSV.dev

**To Reproduce**
Steps to reproduce the behaviour:
1. View a vulnerability record on OSV.dev with an (unlinked) alias to a CVE
2. Attempt to retrieve that CVE via the API
3. Get a 404/Bug not found response from the API

**Expected behaviour**
The user receives some pointers to documentation to assist with interpreting the search failure.

My initial thoughts:

- include a link to https://osv.dev/faq in the message text
- add a new FAQ entry about bugs not found/404s from the API
- add to https://google.github.io/osv.dev/post-v1-query/ about querying by first class vulnerabilities versus aliases (also discussed in #1016)

**Screenshots**

Contrived example:

```
$ GET -s https://api.osv.dev/v1/vulns/CVE-2025-0001
404 Not Found
{"code":5,"message":"Bug not found."}
```