Change all local links to be relative (rebased)

[carnhofdaki]

This is a rebased https://github.com/bitcoin-core/bitcoincore.org/pull/741, nothing added and did not even check if new absolute links need {{ ROOT_PATH }}.

[laanwj]

Looks like there's a lot of HTMLproofer errors in finding the release binaries. I don't know if this is a problem with the generated HTML or the tool.

[raucao]

See https://github.com/bitcoin-core/bitcoincore.org/pull/741#discussion_r562191048 for that unresolved issue from my PR.

[katesalazar]

no longer your can of worms 😂

[maflcko]

Can this be closed?

[carnhofdaki]

Sure. Closing.