Fix links in doc/release-process.md

Summary:
Link to /contrib/seeds/README.md didn't work
Make /doc/gitian-building.md into a link
Make link to /doc/bips.md absolute

Test Plan: I don't know how to test until it reaches GitHub

Reviewers: Fabien, jasonbcox, deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2288