Add basic usage information and fix references

Add basic usage information and fix references

* doc/misc/epa.texi (Top): Add menu entry for new node GnuPG Pinentry.
(Quick Start): Add information on and reference to basic GnuPG
configuration.
(Encrypting/decrypting gpg files): Add usage information.
(GnuPG version compatibility): Update version information.
(GnuPG Pinentry): Add new node.
(Caching Passphrases): Describe mandatory gpg-agent usage for GnuPG
2.0 and later.
(Overview, Encrypting/decrypting gpg files, GnuPG version compatibility)
(Caching Passphrases, Bug Reports): Fix references, terminology,
mark-up, and index entries.  (Bug#64154)