Fix redirects.

The redirects are relative to the current page, so they need `/` to
anchor to the root of the site.

Author: ehuss

book.toml

@@ -44,9 +44,9 @@ curly-quotes = true
 "compiler/diagnostic-codes.html" = "https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-codes.html"
 "compiler/profile-queries.html" = "https://rustc-dev-guide.rust-lang.org/queries/profiling.html"
 "chat/index.html" = "/platforms/index.html"
-"chat/discord.html" = "platforms/discord.html"
-"github.html" = "platforms/github.html"
-"chat/email.html" = "platforms/email.html"
-"chat/zulip/index.html" = "platforms/zulip/index.html"
-"chat/zulip.html" = "platforms/zulip.html"
+"chat/discord.html" = "/platforms/discord.html"
+"github.html" = "/platforms/github.html"
+"chat/email.html" = "/platforms/email.html"
+"chat/zulip/index.html" = "/platforms/zulip/index.html"
+"chat/zulip.html" = "/platforms/zulip.html"
 "platforms/zulip/triagebot.html" = "/triagebot/index.html"