Assume your reference is [this](https://geany.org/manual/dev/hacking.html#filetypes-configuration-file), it doesn't link.
Hacking (and Makefile.am) is for working on Geany source, so you need to get Geany source. An install from a distro won't have source.
But the section I noted above is for adding a builtin filetype, needing code changes in Geany (which you would not be able to make without source anyway) not a custom filetype as discussed in #3232.
So I suspect this is not what you want to do.
Yes Ubuntu LTS is well long term stable, ie it doesn't update to newer versions, those go to the non-LTS first.