Update README

[Gold856]

Description

Try to be slightly more GPL compliant by listing all the dependencies we directly use (Commons Exec is unused by us, so removed.) This also clarifies some info about toolchains and future-proofs the statement about base images by saying it's for all supported coprocessors. Finally, the documentation header was moved up so people are more likely to see it when scrolling through the README on GitHub.

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with settings back to v2025.3.2
• If this PR touches pipeline settings or anything related to data exchange, the frontend typing is updated
• If this PR addresses a bug, a regression test for it is added