build: bump html-proofer from 3.14.1 to 3.15.0 in /docs

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.14.1 to 3.15.0.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.14.1...v3.15.0

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>