Previously, I’ve discussed methods on using Markdown to Confluence generators, one in Groovy, another in Golang. Alternatively, you can use a Markdown plugin for Confluence that will display Markdown inline from an external URL reference.
Basically this is the only zero-cost plugin. Also, it has a lot more users than the expensive ones, so it must be better due to the higher momentum the larger user community garners.
20181113/https://marketplace.atlassian.com/apps/1211438/markdown-macro-for-confluence?hosting=server&tab=overview