Add Spacebin

[lukewhrit]

Thanks for taking the time to suggest an addition to awesome-selfhosted!

To ensure your Pull Request is dealt with swiftly, please check the following (check the boxes [x]):

• [X] Submit one item per pull request. This eases reviewing and speeds up inclusion.
• [X] You have searched the repository for any relevant issues or PRs, including closed ones.
• [X] Any software you are adding is not already listed at any of awesome-sysadmin, staticgen.com, staticsitegenerators.net, dbdb.io.
• [X] The file you are adding is formatted as described in addition.md.
• [X] Demo links should only be used for interactive demos, i.e. not video demonstrations.
• [X] Comments and unused optional fields have been removed.
• [X] The file you are adding uses kebab-case file naming, for example my-awesome-software.yml.
• [X] Values for platform are the main server-side requirements for the software. Don't include frameworks or specific dialects.
• [X] Any software project you are adding to the list is actively maintained.
• [X] Any software project you are adding was first released more than 4 months ago.
• [X] Any software project you are adding has working installation instructions.
• [X] You understand that your Pull Request will be merged ~1 week after approval, to allow for further comments if needed.