Run helm-docs on new PRs

Description The Helm chart documentation gets out of sync on each release (https://github.com/meilisearch/meilisearch-kubernetes/blob/main/charts/meilisearch/README.md reports latest version as 0.2.1 even though it should be 0.2.5), since helm-docs isn't a part of the Github workflows. I propose that we run helm-docs on merging with main or checking that contributors has run the command before it can be merged into main.

Basic example A simple step which verifies that a contributor has run helm-docs:

      - name: Render documentation
        run: |
          export PATH=$PATH:$(go env GOPATH)/bin
          helm-docs
          git update-index -q --ignore-submodules --refresh
          if ! git diff-files --quiet --ignore-submodules -- ; then
            echo Generating documentation caused a change - did you forget to run 'helm-docs' before commit?
            exit 1
          fi

meilisearch / meilisearch-kubernetes

Run helm-docs on new PRs #198