move mercury_login.md to docs folder

[h00die]

While working on other modules I noted this doc was in the wrong folder. Simply move it where it needs to go so I can finish other things.

[bcoles]

Approved, but the documentation format might make h00die sad.

# ./tools/dev/msftidy_docs.rb modules/exploits/windows/imap/mercury_login.md 
modules/exploits/windows/imap/mercury_login.md - [WARNING] Docs should start with ## Vulnerable Application
modules/exploits/windows/imap/mercury_login.md - [WARNING] Missing Section: ## Verification Steps
modules/exploits/windows/imap/mercury_login.md - [WARNING] Missing Section: ## Options
modules/exploits/windows/imap/mercury_login.md - [WARNING] H2 headings in incorrect order. Should be: Vulnerable Application, Verification Steps, Options, Scenarios
modules/exploits/windows/imap/mercury_login.md:1 - [WARNING] Line too long (159). Consider a newline (which resolves to a space in markdown) to break it up around 140 characters.
modules/exploits/windows/imap/mercury_login.md:5 - [WARNING] Line too long (250). Consider a newline (which resolves to a space in markdown) to break it up around 140 characters.

[h00die]

it'll join a LONG list of docs which are not up to spec

[bcoles]

Added rn-no-release-notes as we don't have a release notes label for docs. I delegate responsibility for the release notes to whoever disagrees.