add `rendered_path` in the template

[pinage404]

pass the rendered path in the template, as suggested here https://github.com/rust-lang/mdBook/issues/1238#issuecomment-636270351

which allows to add the canonical URL https://gitlab.com/pinage404/git-gamble/-/commit/c7c2e601f9d42f1d2f88e81654787c4d0ba82545#4e2dfdde34a65a1bb4b51a5effb6635b41af0fbc_59_60

the intention of this PR is just to help the subject to advance, i don't care if it is not merged because prefered to another solution

the intention of this PR is just to help the topic move forward, i don't care if it is not merged because it is preferred to another solution