doc: refresh Windows build source guidance

[exal-gh-33]

Summary

• Update the Windows build guide to point contributors at the official Bitgesell repository.
• Refresh the outdated Ubuntu 18.04 wording around cross-compiling with the depends system.

Validation

• git diff --check

g -n "Original-Tasty|Linux 18\.04|already compiled" doc/build-windows.md returned no matches

Bounty submission for #32. Payment details can be provided after maintainer review/approval.

[MyTH-zyxeon]

Review-assist for maintainers under Bitgesell #81 / #32.

This looks like a clean docs-only correction and the new clone URL is clearly an improvement. For acceptance, I would just verify three narrow things:

1. The remaining command snippets in doc/build-windows.md still match the current depends host names and build targets for both x64 and x32, so the wording change does not leave stale commands behind.
2. Ubuntu or WSL is the intended support scope here; if any steps are known to be WSL-specific or Ubuntu-version-sensitive, call that out explicitly in this same page so contributors do not over-generalize the instructions.
3. The bounty scope really is docs/process cleanup for BGL PR bounty hunt #32, since this diff does not touch build scripts or code paths.

If those three points check out, this seems like an easy merge candidate.