feat: added underlying terraform provider version to readme

[Maed223]

Fixes: hashicorp/terraform-cdk#2680

Includes underlying provider version in README as read from src/version.json.

Side benefit of this is using the actual underlying provider version to build the link to the terraform registry, instead of an approximate version as before.

Found there to be some ambiguity around when/if src/version.json would be present as you can see here:

    // set provider name and version from version.json (if it exists yet)
    const versionFile = path.resolve(project.outdir, "src/version.json");

    let fullyQualifiedProviderName = "<unknown>";
    let actualProviderVersion = "<unknown>";
    if (existsSync(versionFile)) {
      const map = JSON.parse(readFileSync(versionFile, "utf8"));
      if (Object.keys(map).length !== 1) {
        throw new Error(
          "version.json must have exactly one entry. specifying multiple providers is not supported"
        );
      }
      const [fqpn, version] = Object.entries(map)[0];
      fullyQualifiedProviderName = fqpn;
      actualProviderVersion = version as string;
    }

As such I've made sure to default to the current state of the README is the event it isn't present.

[github-actions]

I'm going to lock this pull request because it has been closed for at least 7 days. This helps our maintainers find and focus on the active issues. If you've found a problem that seems related to this change, please open a new issue so we can investigate further.