Documentation sources are not linted in CI

Documentation pull requests pass CI with formatting and Markdown quality defects which should be caught before review. The docs CI workflow only builds the site ([`.github/workflows/docs-pull-request.yml`](https://github.com/Flagsmith/flagsmith/blob/main/.github/workflows/docs-pull-request.yml)); the existing `lint` Makefile target ([`docs/Makefile`](https://github.com/Flagsmith/flagsmith/blob/main/docs/Makefile)) and the repository pre-commit prettier hook ([`.pre-commit-config.yaml`](https://github.com/Flagsmith/flagsmith/blob/main/.pre-commit-config.yaml)) are never run by CI. As a result, the current documentation sources already fail formatting on 151 files and carry thousands of Markdown lint findings.

## Acceptance criteria

- [ ] All current documentation sources under `docs/` pass formatting and Markdown lint checks.
- [ ] A documentation pull request that introduces a formatting or Markdown lint defect fails CI.

Start with .github/workflows/docs-pull-request.yml, then inspect the lint target in docs/Makefile and the prettier hook in .pre-commit-config.yaml. Run the existing documentation lint checks, resolve the current formatting and Markdown findings under docs/, and verify that a documentation pull request with a lint defect fails CI.

Written by the indexing model from the issue text.