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- remove install.sh / install.ps1 / install_bsd.sh (superseded by dcrup)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

docs/getting-started/installation.mdx

@@ -45,10 +45,12 @@ After `self-install`, the command is **`dcrup`** (no `.sh` / `.ps1`): shims live
 ### Common dcrup commands
 
 ```sh
-dcrup install stable              # latest stable prebuilt
+dcrup install stable              # latest stable prebuilt (auto: cf → gh → sf → ya)
 dcrup install 0.8.2               # pin → 0.8.2@stable
 dcrup install 0.8.2@dev
 dcrup install stable --libc musl  # Linux: musl asset (default: gnu)
+dcrup install stable --mirror cf  # CloudFlare CDN
+dcrup install stable --mirror ya  # Yandex CDN (RU fallback)
 dcrup install stable --build      # cargo build --features archive
 dcrup install night               # always build from branch dev HEAD
 dcrup default 0.8.2
@@ -60,6 +62,22 @@ dcrup which
 
 Layout: `~/.dcr/toolchains/<id>/dcr` and `~/.dcr/bin/dcr` → active version.
 
+### Download mirrors
+
+Prebuilt installs try **cf → gh → sf → ya** unless you pin a host.
+
+| Code | URL | Notes |
+|------|-----|--------|
+| `cf` | `https://cdn-global.dcr-tool.ru/<tag>/<asset>` | CloudFlare CDN (worldwide) |
+| `gh` | `https://github.com/dexoron/dcr/releases/download/<tag>/<asset>` | GitHub Releases |
+| `sf` | `https://sourceforge.net/projects/dcr-tool/files/<tag>/<asset>/download` | SourceForge |
+| `ya` | `https://cdn-ru.dcr-tool.ru/<tag>/<asset>` | Yandex CDN (RU fallback) |
+
+```sh
+dcrup install stable --mirror auto   # default
+export DCRUP_MIRROR=cf               # persist for this shell
+```
+
 ---
 
 ## Arch Linux (AUR)