docs: update gitian building docs

Github-Pull: #5512
Rebased-From: dd14a0e799b6552ae85aaab2b0b48f788c578aea
Do not use libgmp automatically in libsecp256k1
Merge pull request #5531
Preliminary release notes for 0.10
CLIENT_VERSION_IS_RELEASE → true
doc: add section about watch-only addresses to release notes