nginx-repo.cert -> nginx-repo.crt - F5 WAF on NGINX

[ald8]

The NGINX repositories for NGINX and F5 WAF for NGINX packages contain a hardcoded certificate file name nginx-repo.crt. Consequently, it is necessary to modify the nginx-repo.cert file to nginx-repo.crt throughout the affected docpages, or at the very least at the following locations:

• https://github.com/nginx/documentation/blob/main/content/includes/waf/install-build-image.md
• https://github.com/nginx/documentation/blob/main/content/waf/install/docker.md

[mjang]

Fix in progress in #1398