Ignore auto generated files

[abdonrd]

What do you think about ignore the generated files like docs, analysis.json, etc? Easier to see the PRs.

[vlukashov]

It would be a nice improvement if these auto-generated files do not appear in PRs. Currently they are part of the repo because its GitHub pages are set up to render the content of the /docs folder.

If there is any way to keep GitHub pages up-to-date but exclude the autogenerated files from PR reviews, let's do that.

One idea how to do that was to use the gh-pages branch (https://github.com/vaadin/vaadin-router/issues/95) instead of /docs. That would require some build scripting magic to make sure the yarn docs command still works.