Improve documentation for setting up custom certificates

This issue is meant to capture suggestions for improvements to the documentation as stated in #27067 (comment 424889900), namely:

• Make top section of https://docs.gitlab.com/runner/configuration/tls-self-signed.html more generic
• Add LFS self-signed error example:

  LFS: Get https://object.xxx.com/lfs-dev/c8/95/a34909dce385b85cee1a943788044859d685e66c002dbf7b28e10abeef20?X-Amz-Expires=600&X-Amz-Date=20201006T043010Z&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=svcgitlabstoragedev%2F20201006%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-SignedHeaders=host&X-Amz-Signature=012211eb0ff0e374086e8c2d37556f2d8ca4cc948763e90896f8f5774a100b55: x509: certificate signed by unknown authority

• Example using Kubernetes volumes on https://docs.gitlab.com/runner/configuration/tls-self-signed.html#trusting-the-certificate-for-the-other-cicd-stages - looks like I need to use format of https://docs.gitlab.com/runner/executors/kubernetes.html#using-volumes?
• Add details to tls-ca-file about how you might also need https://docs.gitlab.com/runner/configuration/tls-self-signed.html
• FF_USE_DIRECT_DOWNLOAD on https://docs.gitlab.com/runner/configuration/feature-flags.html link back to https://docs.gitlab.com/runner/configuration/tls-self-signed.html#trusting-the-certificate-for-the-other-cicd-stages for how to fix issue
  • This is currently not possible since the documentation is generated from helpers/featureflags/flags.go and we can't add absolute links to feature-flags.md (will cause a linter error).
• Kubernetes runner helm chart seems to be doing some magic to add certs to system? gitlab-org/charts/gitlab-runner!202 (diffs)