Revert "Fixed link - also using markdown instead of link to"

This reverts commit 3ec4f57b5687a0cb5b5fcb744486540c98a0b1cb.
Add the checkout flow customization guide
Add addresses and taxation to sidebar
Fix code examples in activators and promotions guides
Fix link to preferences in checkout guide
Fix usage of shell in place of bash in navigating+about guides
Convert preferences guide to new format
Placeholder for new getting help guide
Use bash for code block, etc.
Renamed to shorter MD filename extension
Ported over deployment service guide
Add AppConfiguration preferences to preferences guide
Sort developer sidebar in alphabetical order
Ported application process and merged with deployment service
Merged server config into deployment service guide
Flesh out Getting Help guide
Port over testing guide
Port over security guide
Add mention of security list to GitHub part of getting help guide
Mention in contributing guide how PRs are 'merged' from GitHub