docs: fixed formatting and diffs from actual implementation

[hwakabh]

Describe the change you are making here!

• replaced vagrant add with vagrant box add in docs, since there is no subcommands with latest packer
• fixed markdown styles
• replaced prefix of box from packer- to output in docs, since it had been modified in implementation

Please include tests. We recommend looking at existing tests as an example.

24-11-19 22:36:17 git/packer-plugin-vagrant [docs/fix-builder-vagrant] % make test
?       github.com/hashicorp/packer-plugin-vagrant      [no test files]
?       github.com/hashicorp/packer-plugin-vagrant/version      [no test files]
ok      github.com/hashicorp/packer-plugin-vagrant/builder/vagrant      1.468s
ok      github.com/hashicorp/packer-plugin-vagrant/post-processor/hcp-vagrant-registry  2.499s
ok      github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant       2.272s
ok      github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant-cloud 2.830s

If your PR resolves any open issue(s), please indicate them like this so they will be closed when your PR is merged:
N/A

[hashicorp-cla-app]

All committers have signed the CLA.

[lbajolet-hashicorp]

Hey @hwakabh,

Thanks for the PR, everything looks good to me, we can merge this right now, thanks!