docs(git-clone): add example for cloning multiple repositories

[blinkagent]

Description

git-clone takes a single url, so cloning several repositories requires for_each on the module block. That pattern was not documented, so this adds a Clone multiple repositories section to the module README covering:

• the basic for_each form, with the start_count check moved into the for_each expression (Terraform does not allow count and for_each on the same block)
• per-instance isolation of scripts and logs under ~/.coder-modules/coder/git-clone/<folder_name>
• consuming the per-key outputs, e.g. for_each = module.git_clone to create one code-server/coder_app per repository
• a warning that two repositories with the same basename collide unless folder_name is set
• a map-of-objects variant for per-repository branch_name / folder_name / extra_args

Documentation only, no module version bump.

Testing

Validated against the local module with Terraform 1.9.8, cloning coder/coder and coder/registry in one template:

• terraform apply succeeds with for_each; each instance produces distinct repo_dir, folder_name, and clone_url outputs
• running both generated coder_script payloads concurrently clones both repositories successfully, with separate scripts/ and logs/ directories per instance
• re-running both scripts is idempotent (already exists and isn't empty, skipping clone!)
• the map-of-objects variant with folder_name = each.key, branch_name, extra_args = ["--depth=1"], and a post_clone_script works; each instance writes its own post_clone.sh and post_clone.log
• for_each = module.git_clone on a downstream resource resolves correctly per key
• prettier --check passes on the README

Created on behalf of @matifali

[matifali]

Closing in favour of #1171