Link to older releases

[visr]

Currently https://ribasim.org/changelog shows the release notes of all releases, but https://ribasim.org/install only links to the assets of the latest release. Today I had to guide some users on how to find old releases from the docs, which is not straightforward:

• click the GitHub icon at the top of ribasim.org
• go to https://github.com/Deltares/Ribasim/releases
• find the version you want
• open the assets dropdown at the bottom
• download

Perhaps we should have a little section on old releases in https://ribasim.org/install and/or add download links in the changelog page.