fix(pdd): exclude some doc files
We noticed that sometimes we can use the keyword `@todo` in `CHANGELOG.md`, `CODE_OF_CONDUCT.md`,
`CONTRIBUTING.md` without it being a puzzle. But `pdd` will fail on that saying that it doesn't
match to puzzle format.
We decided to exclude them all from pdd checking.
Cc: (commit)[https://github.com/artipie/artipie/commit/b281fbd957752567387e707f090cfd5b52fc931a#commitcomment-65919004]
--exclude .idea/**/*
--exclude examples/**/*
--exclude examples/helm/tomcat-0.4.1.tgz
+--exclude CHANGELOG.md
+--exclude CODE_OF_CONDUCT.md
+--exclude CONTRIBUTING.md
--rule min-words:20
--rule min-estimate:15
--rule max-estimate:90
