Use Discourse's search API to find new location of each page.

Done using this script:
```python

import yaml
import requests
import sys
import time

BASEURL = "https://forum.qubes-os.org"

def main(path):
    with open(path) as f:
        frontmatter = yaml.safe_load_all(f)
        frontmatter = list(frontmatter)[0]
    # rate limit of 20req/min...
    time.sleep(3)
    # https://docs.discourse.org/#tag/Search/operation/search
    r = requests.get(f"{BASEURL}/search.json", params={
            "q": f"{frontmatter['title']} #guides in:title"})

    r.raise_for_status()
    search = r.json()
    if 'topics' not in search or not search['topics']:
        raise Exception(f"No topic found for \"{frontmatter['title']}\"")
    result = search['topics'][0]
    frontmatter["redirect_to"] = f"{BASEURL}/t/{result['slug']}/{result['id']}"
    with open(path, "w") as f:
        f.write("---\n")
        f.write(yaml.dump(frontmatter))
        f.write("---\n")

if __name__ == "__main__":
    try:
        main(sys.argv[1])
    except Exception as e:
        print(f"{sys.argv[1]}: {e!s}")
```

And called this way:

    find external/ -type f -exec update-community-redirect {} \;

Not all pages were found:

    external/building-guides/building-archlinux-template.md: No topic found for "Building Arch Linux template"
    external/building-guides/building-non-fedora-template.md: No topic found for "Building non-Fedora template"
    external/configuration-guides/multimedia.md: No topic found for "How to make a multimedia template"
    external/configuration-guides/tips-and-tricks.md: No topic found for "Tips and tricks"
    external/customization-guides/fedora-minimal-template-customization.md: No topic found for "Fedora minimal template customization"
    external/customization-guides/removing-templatevm-packages.md: No topic found for "Removing template packages"
    external/customization-guides/windows-template-customization.md: No topic found for "Windows template customization"
    external/os-guides/netbsd.md: No topic found for "How to create a NetBSD qube"
    external/os-guides/pentesting/blackarch.md: No topic found for "How to create a BlackArch qube"
    external/os-guides/pentesting/kali.md: No topic found for "How to create a Kali Linux qube"
    external/os-guides/pentesting/ptf.md: No topic found for "How to create penetration testers framework (PTF) qube"
    external/privacy-guides/tails.md: No topic found for "Running Tails in qubes"
    external/security-guides/multifactor-authentication.md: No topic found for "Multifactor authentication"
    external/troubleshooting/sony-vaio-tinkering.md: No topic found for "Sony Vaio tinkering"