Modify workflow triggers and status badge

[ScottBrenner]

Regenerated from https://github.com/opencontainers/image-spec/actions/workflows/docs-and-linting.yml, doesn't render currently.

[sudo-bmitch]

It looks like this badge is broken because we don't run the workflow on a push to main, only on PRs. I don't think the badge should show a fail if we get a bad PR. Instead we should either remove the badge, or (my preference) update the workflow to run on a push to main. E.g.

on:
  push:
    branches: [ 'main' ]
    tags: [ 'v*.*.*' ]
  pull_request:
    branches: [ 'main' ]

[ScottBrenner]

Updated the badge to display the main branch, modified workflow trigger as suggested too ✅