Add trailing slashes to permalinks

[sjaks]

To ensure urls with and without trailing slasles take to the correct
page, include a slash at the end of permalinks

Kudos to @Moppa5 for finding a possible solution quickly

bundle install --path vendor/bundle
bundle exec jekyll serve

curl -L http://127.0.0.1:4000/docs/get-started/available-commands
curl -L http://127.0.0.1:4000/docs/get-started/available-commands/

[Moppa5]

As mentioned curls work and correct pages are served when required. I tested by making some browsing on local environment and it works normally. As far as I'm concerned feel free to merge.