Add support for index-path and pages-index-path

[powerfooI]

As mentioned in #183, it seems no way to customize index-path and pages-index-path in current chart-releaser-action.

What I want to achieve is that chart-releaser-action reads and updates index.yaml in a sub folder like docs/static/index.yaml instead of project root. The docs/ may be a nodejs sub project aiming to generate static site to gh-pages branch.

Thank you so much for reviewing!

[masayag]

@davidkarlsen can you have a look at this PR?

[powerfooI]

@masayag I have release a distribution here and tested it successfully in our project. I added index-path, pages-index-path and pr parameters. Maybe we could take it as an ephemeral choice.