feat: add download mirrors and drop legacy install scripts

Introduce `--mirror` and `DCRUP_MIRROR` so `dcrup` can fetch prebuilt
binaries from multiple CDNs. Default `auto` tries cf → gh → sf → ya
and uses the first successful download.

- dcrup.sh / dcrup.ps1: mirror chain, CLI flag, env override
- remove install.sh / install.ps1 / install_bsd.sh (superseded by dcrup)
- GitHub Actions: upload releases to Cloudflare R2 next to Yandex S3
- docs: README, FAQ, changelog, installation, env vars, contributing

FAQ.md

@@ -61,8 +61,19 @@ Available installation methods may vary by project version.
 Recommended path:
 
 - check instructions in `README.md`;
+- use `dcrup install stable` (default mirror chain: **cf → gh → sf → ya**);
+- pin a host with `--mirror cf|gh|sf|ya` or `DCRUP_MIRROR` if needed;
 - if there is no prebuilt package for your platform, build from source using `cargo build --release`.
 
+## How do I choose a download mirror?
+
+`dcrup` defaults to `--mirror auto`: CloudFlare CDN, then GitHub, then SourceForge, then Yandex CDN.
+
+```sh
+dcrup install stable --mirror cf
+export DCRUP_MIRROR=ya
+```
+
 ## How can I get started quickly?
 
 Minimal workflow: