Fix registration error info typo
Add .adoc file extension for AsciiDoc

Add .adoc file extension for AsciiDoc as it's a very popular extension to use and the one recommended by the Asciidoctor project.
Switched line endings to LF for system_hooks.md

It was checked in with Windows style line endings.
Merge pull request #6535 from docwhat/sys-hooks-eols

Switched line endings to LF for system_hooks.md
Merge pull request #6457 from lol768/patch-1

Fix registration error info typo
Merge pull request #6469 from mojavelinux/master

Add .adoc file extension for AsciiDoc
typo fix
Merge pull request #6568 from skv-headless/patch-1

typo fix
Fix `/:username.keys` response content type

Currently this method responds with `text/html`. It is kind of unusable
if you open it in a browser. The browser thinks it is HTML and renders
it as HTML, meaning new lines are dropped. So it's very hard to
distinguish where the key starts and where it ends.

This commit changes the content type header to `text/plain`.