As @1440000bytes already mentioned, Bitcoin doesn't have an official website, although there are numerous sites run by various individuals. Some projects have their own websites such as bitcoincore.org.
The final version of the release will be tagged and signed by one of the maintainers whose keys are listed in bitcoin/blob/master/contrib/verify-commits/trusted-keys. Release candidates are then built by many volunteers from the source code in parallel in our deterministic build process to compare that they create the same binary. The builders' keys, attestations, and instructions how to verify them are found in the release process document. You can find a description of the process here: https://bitcoin.stackexchange.com/a/110517/5406
Older versions of Bitcoin Core releases were signed with the release signing key.
If so inclined, you will find that many of these keys, especially the release signing key, are attested to by other Bitcoin Core contributors. If this does not suffice, you could put the key into your web of trust by attending a key signing party at a Bitcoin conference.