Converting some of the rst documentation to markdown.  Attempted to
preserve whitespace and line breaks to minimize cosmetic change.

Signed-off-by: Aaron Raimist <aaron@raim.ist>

It looks like CAPTCHA_SETUP.rst contains information relevant to an old version of Synapse, but Synapse now has a different approach to configuring use of the X-Forwarded-For header.

this is rst so name it accordingly

Update CAPTCHA_SETUP (it continues to ignore fallback, but I guess I should fix it so that doesn't need the key in two different places)

Add original, unmodified CAPTCHA-SETUP from the webclient repo before modifying (captcha setup is now purely on the HS).