Include Strings guide with the others.

Three small changes:

1. Re-organize headers in the Strings guide so they show up correctly.
2. build the strings guide with the other docs
3. include the strings guide in the list of guides
auto merge of #15782 : steveklabnik/rust/string_guide_link, r=cmr

Three small changes:

1. Re-organize headers in the Strings guide so they show up correctly.
2. build the strings guide with the other docs
3. include the strings guide in the list of guides