Warn about input errors with object inputs, remove DNS section

[Nuru]

what

• Warn about input errors being silently ignored with object inputs
• Remove DNS section from Terraform Best Practices
• Incorporate some changes from fixed typos in docs #584 and update some others, thanks @whyevenquestion1t
• Enhance site builder to render README.yaml of sub-modules when present

why

• Non-obvious behavior can lead to serious consequences
• Recommendations about DNS configuration are not relevant to Terraform
• Supersedes and closes fixed typos in docs #584, which is not completely ready to merge and is nearly a year old
• Submodule README.md is expected to be plain. If it is generated by README.yaml, then parse it like we do for top-level modules so we do not get the extra stuff that breaks Docusaurus.