Add list of proposed release dates for July - December 2026 to public-facing release calendar in docs

Add the list to this page: https://github.com/OvertureMaps/docs/blob/main/docs/release-calendar.mdx

There's no reason to maintain an internal and external list of release dates. I recommend we change the heading on [this page](https://docs.overturemaps.org/release-calendar/) to "Proposed Release Schedule" and include all the dates through the end of the year. 