Changed sha1 into bcrypt
[#3356] Fixing ticks that don't work inside a bold and removing extra details in intro area that aren't necessary (you'll need to read the cookbook entry for more details anyways)
[#3356] Changing to use the bcrypt algorithm

Also removing one example (now possible with using bcrypt from the beginning) and related tweaks.
[#3356] Moving serialize and Equatable logic further down for clarity
[#3356] Clarifying when you need a salt

Also filling in other details related to using BCrypt
[#3405] Tweaks thanks to @WouterJ and @xabbuh
Merge pull request #3405 from symfony/security_fix_extended

Changed sha1 into bcrypt continued...