Clarify licenses of member logos etc

https://github.com/matrix-org/matrix.org/blob/bfcb1b996398b944ff70df3918147b4188789550/LICENSE.md?plain=1#L4

While we can probably assume we are safe to use the logos ourselves, I think we technically need to put an exemption here.

Acceptance Criteria

• update documentation in the repo: we generally take contributions under apache 2.0 or cc-by-sa 4.0 respectively. we track licenses using https://reuse.software/. some exceptions are made, e.g. for members logos and similar third party assets, and thus indicated via REUSE-compliant annotations. we also allow ecosystem projects to submit assets under an individual license, provided it is compatible with the rest of our repo. note that trademarks are a separate concern.
  • LICENSE.md
  • CONTRIBUTING.md (specifically the signoff, may need alignment with legal team)
  • maybe CONTENT.md?
• implement https://reuse.software/ in CI, e.g. https://github.com/fsfe/reuse-action. this could be done so the checker only applies incrementally, e.g. only to files that a change actually touched, so at least we force ourselves to track licenses properly from here on forward.
• clean up the loose licenses in root vs the LICENSE folder
• apply the member logo license https://github.com/matrix-org/matrix.org/issues/2706#issuecomment-4458925073 to members logos via REUSE
• copy the same over to the conference website

Review LICENSE.md and CONTRIBUTING.md first, including the signoff language and the existing license layout; check whether CONTENT.md also needs alignment. Then inspect the proposed reuse-action integration and the member-logo annotations. Done means the repository documents its exceptions, tracks them with REUSE, and the same license treatment is applied to the conference website.