Document use of precompiled proxies and compiler version for proxies

[ericglau]

The Hardhat Upgrades plugin uses precompiled proxy bytecodes that are packaged in the NPM package @openzeppelin/upgrades-core.

Document this behavior to make it clear that:

1. The proxy compiler version is independent of the compiler version specified in the user's project,
2. Different versions of @openzeppelin/upgrades-core may use different compiler versions and different versions of the proxies from OpenZeppelin Contracts, and
3. If users want the same proxy bytecodes deployed to different chains, they should consistently use the same versions of @openzeppelin/hardhat-upgrades and @openzeppelin/upgrades-core for those deployments, and for source code verification.