Add download details, including download signature for beta.

Not pushing to front page until we have proper download links. Will
probably adjust somewhat for the proper release. The location of the
signature file will almost certainly change.

generated/state/feed-pages.txt

@@ -2393,12 +2393,13 @@
 "/doc/libs/1_61_0_b1/
 -download
 -download_basename
+"boost_1_61_0_b1
 -final_documentation
 "/doc/libs/1_61_0/
 -id
 "version_1_61_0
 -last_modified
-=1459754607
+=1459756918
 -location
 "users/history/version_1_61_0.html
 -notice
@@ -2411,7 +2412,7 @@
 "      New Libraries: Updated Libraries:
 "
 -qbk_hash
-"f7c30f42d2f96897f518d05bdf32ae86838620f03cd422be07386c5219e8422a
+"bc34edcdea6f44d2c7fb152559d1cd5771c7ef98a33d44979816d8d407959ece
 -release_status
 "dev
 -title

generated/state/rss-items.txt

@@ -31371,7 +31371,7 @@
 "  </div>
 "</description></item>
 -last_modified
-=1459754607
+=1459756918
 -quickbook
 "feed/history/boost_1_61_0.qbk
 )