Merge #11401: doc: move gitian building to external repo

fa082b4 doc: move gitian building to external repo (MarcoFalke)

Pull request description:

  The guide was moved to [the Bitcoin Core documentation repository](https://github.com/bitcoin-core/docs/blob/master/gitian-building.md).

  It now includes not only instructions to run on a debian host, but also on
  any recent fedora host.

  This is an ongoing effort to reduce the fraction caused by incoming
  pull requests to docs (usually fixups, typo fixes, version bumps).

  This is especially important for documentation that is independent
  of any release cycles and should thus be up-to-date regardless
  of a tag on a non-master branch.

  Finally, fetching a shallow copy of the source is less heavy,
  as many binary png files are moved out of the repo.

Tree-SHA512: c88a11667826d44f90af41e50f9e8d5b3132ae83b278597f13be694872212e7eff99fd877feec0308e00259c69fd4e0ebaf3ff5ab45e08a1e41336aa505bb72b

Author: laanwj