docs: Add basic document on matching rules support for header fields

[m-guesnon-pvotal]

Related Issue or Design Document

Related source code pull request can be found here: https://github.com/ory/oathkeeper/pull/967

Checklist

• [x] I have read the contributing guidelines and signed the CLA.
• [ ] I have referenced an issue containing the design document if my change introduces a new feature.
• [x] I have read the security policy.
• [x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got green light (please contact [email protected]) from the maintainers to push the changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added necessary documentation within the code base (if appropriate).

Further comments

[CLAassistant]

All committers have signed the CLA.

[vinckr]

added the upstream label to make clear this is waiting for the PR in oathkeeper.