docs: Update upgrade guide

* Update Caddy 2 plugin instructions

The website said there is no interactive download page, but there is
now. Updated documentation to reflect this.

* Fix broken link to main.go

* Change xcaddy link to current build docs

src/docs/markdown/v2-upgrade.md

@@ -76,6 +76,7 @@ IP addresses and localhost domains will be issued certificates from a [locally-t
 The storage structure of certificates and ACME resources has changed. Caddy 2 will probably obtain new certificates for your sites; but if you have a lot of certificates you can migrate them manually if it does not do it for you. See issues [#2955](https://github.com/caddyserver/caddy/issues/2955) and [#3124](https://github.com/caddyserver/caddy/issues/3124) for details.
 
 
+
 ## Command line
 
 The `caddy` command is now `caddy run`.
@@ -100,6 +101,7 @@ The [v2 Caddyfile](/docs/caddyfile/concepts) is very similar to what you're alre
 
 ⚠️ **Be sure to read into the new directives!** Especially if your config is more advanced, there are many nuances to consider. These tips will get you mostly switched over pretty quickly, but please read the full documentation for each directive so you can understand the implications of the upgrade. And of course, always test your configs thoroughly before putting them into production.
 
+
 ### Primary changes
 
 - If you are serving static files, you will need to add a [`file_server` directive](/docs/caddyfile/directives/file_server), since Caddy 2 does not assume this by default. Caddy 2 does not sniff MIME by default, either, for security reasons; if a Content-Type is missing you may need to set the header yourself using the [header](/docs/caddyfile/directives/header) directive.
@@ -147,6 +149,7 @@ browse /subfolder/
 file_server /subfolder/* browse
 ```
 
+
 ### errors
 
 Custom error pages can be accomplished with [`handle_errors`](/docs/caddyfile/directives/handle_errors).
@@ -341,6 +344,7 @@ status 404 /secrets/
 respond /secrets/* 404
 ```
 
+
 ### templates
 
 The overall syntax of the [`templates`](/docs/caddyfile/directives/templates) directive is unchanged, but the actual template actions/functions are different and much improved. For example, templates are capable of including files, rendering markdown, making internal sub-requests, parsing front matter, and more!
@@ -376,17 +380,17 @@ If you need a custom service file, base it off of ours. It has been carefully tu
 See [install instructions](/docs/install#linux-service) for details.
 
 
+
 ## Plugins
 
 Plugins written for v1 are not automatically compatible with v2. Many v1 plugins are not even needed in v2. On the other hand, v2 is way more easily extensible and flexible than v1!
 
 If you want to write a plugin for Caddy 2, [learn how to write a Caddy module](/docs/extending-caddy).
 
+
 ### Building Caddy 2 with plugins
 
-Caddy 2 does not (yet) have a public build server and interactive download page like v1 did. We're working on it. In the meantime, our [builder tool](https://github.com/caddyserver/xcaddy) may be helpful. It simply automates the instructions in Caddy's [main.go](https://github.com/caddyserver/caddy/blob/v2/cmd/caddy/main.go) file.
-
-We'll also be working on the new website some more so that plugins can be registered and indexed and easily found.
+Caddy 2 can be downloaded with plugins at the [interactive download page](https://caddyserver.com/download). Alternatively, you can [build Caddy yourself](https://caddyserver.com/docs/build) using `xcaddy` and choose which plugins to include. `xcaddy` automates the instructions in Caddy's [main.go](https://github.com/caddyserver/caddy/blob/master/cmd/caddy/main.go) file.
 
 
 ## Getting help