update ReadTheDocs with example app documentation

[jbibla]

blocked by #335

[jaekwon]

@zramsay

[zramsay]

• rewrite intro in overview.rst
• replace security/framework overview with guide.md and design.md, respectively.
• update the above to .rst
• see README in examples/basecoin
• delete roles
• key management => move to staking directory
• key management => re-write for gaia
• delete basecoin-plugin => replace with extensions
• re-write basecoin/extensions.rst (document the extensions #446)
• hold IBC for now
• update the glossary (s/Modules/handlers+mappers+stores/g) + add Tx (signed message) & Msg (update the glossary #447)
• explain the ante handler (AnteHandler currently requires PubKey to be included in Signature & document AnteHandler #434)
• IPC get rid of
• get rid of permissions (s/perms/object capabilities+obj cap keys/g) (remove permissions from documentation #448)
• move replay protection out of glossary into x/auth module
• get rid of term middleware/isolation
• delete stdlib
• remove countercli and that stuff (refer to correct binaries in docs #449)
• update the diagrams (create diagrams for the documentation #450)

[zramsay]

opened specific issues for what's left.