final touches before a stable release

[lajp]

After this PR we should be able to bump the version of this project to a stable 1.0.0 and focus on new projects.
Things I'd like to address:

• Only allow supported attachment files
• Implement ratelimits for POST /invoices
• Document the API and make sure everything makes sense
• Return "OK" from /health
• Validate IBAN account numbers
• Add the filenames as labels to the generated pdf to the start of each attached pdf
• (?) support multiple attachment with the same filename
• Check for redundant dependencies
• Return error on failed mail send
• Improve logging and error handling to the level of a stable product
• Write tests for the POST /invoices path
• Make everything (to a reasonable extent) configurable either through a .toml file or through env variable
• Prevent unnecessary cloning of attachment files
• Update README and add build instructions
• Bump version to 1.0.0

Others are also welcome to contribute to this branch either directly or through a PR