feat: include robots.txt to prevent search engine crawling

[adan89lion]

Is your feature request related to a problem? Please describe.
I am hosting MicroBin for personal and family use, but I am afraid that some PII/sensitive data would be leaked if robots.txt is not implemented. Sites like Wayback Machine would also archive any possible pastes.

Describe the solution you'd like
Add an environmental variable to prevent search engine indexing/web archiving crawlers.

Describe alternatives you've considered
N/A

Additional context
N/A