docs: Fix broken anchor links in README (mostly terraform)

[Montana]

Hey all,

This fixes 10 broken internal anchor links in README.md. No content changes, only link targets.

When the hooks were renamed from terraform_* to tofu_* during the fork from pre-commit-terraform, the section headings in the README were updated but several links pointing to them were not. As a result, the "Hook notes" links in the "Available Hooks" table and one table-of-contents entry currently lead nowhere.

Changes

• Updated 8 "Hook notes" links in the Available Hooks table to point to the
  existing #tofu_* headings instead of the old #terraform_* anchors:
  tofu_docs, tofu_docs_replace, tofu_docs_without_aggregate_type_defaults,
  tofu_fmt, tofu_providers_lock, tofu_tflint, tofu_tfsec, tofu_trivy,
  and tofu_wrapper_module_for_each
• Fixed the TOC entry "Download OpenTofu modules from private GitHub repositories"
  (#download-tofu-modules-... → #download-opentofu-modules-...) to match the
  actual heading slug

Verification

Checked every internal ](#...) link in the README against GitHub-generated heading anchors, zero broken links remain after this change.

[kvendingoldo]

lgtm! please, fix DCO, after that we'll merge it

[Montana]

Hi @kvendingoldo,

Signed off on it and DCO is successful via:

git rebase HEAD~1 --signoff

Pushed with lease.