fix: made bucket name a var. Update docs

[bzarboni1]

ISSUE

[#100] - [Bug] GCP bucket name is hard-coded but must be globally unique

When running the bootstrap layer for a new organization the following error occurred:

│ Error: googleapi: Error 409: The requested bucket name is not available. The bucket namespace is shared by all users of the system. Please select a different name and try again., conflict
│ 
│   with module.github_gcloud_oidc.google_storage_bucket.bucket,
│   on .terraform/modules/github_gcloud_oidc/modules/github-gcloud-oidc/storage.tf line 2, in resource "google_storage_bucket" "bucket":
│    2: resource "google_storage_bucket" "bucket" {
│ 

The storage bucket name is hardcoded as github-tf-state-bucket, which means only the first org to use the toolkit can successfully setup their org.

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