[lektor] software latest version macros for documentation to be always updated

While writing docs we have some strings that change in each release. Having some sort of macro will help documentation stay updated and the editors will have more time to improve other things.

Latest versions for software

One nice macro should give, in a string, the last Tor Browser version, for example in the string: tor-browser-linux64-10.0.14_zh-CN.tar.xz it will be 10.0.14.

So, we could write 'download Tor Browser i.e. tor-browser-linux64-[!latest_tbb]_zh-CN.tar.xz' or 'Are you using the current Tor Browser ([!latest_tbb])?'.

The most clever thing will be to take this information from the corresponding databags (https://gitweb.torproject.org/project/web/tpo.git/tree/databags/ ) so we dont risk to become outdated, and we dont add more tasks to the release process.