Fundraising tech/notes/Draft:Documentation overhaul

Brainstorming: topics to include

 * FR-Tech source repositories and codebases, what they're used for, where they're deployed, how they're used, links to related documentation pages, links to upstream repos (if applicable)
 * Civicrm: Triggers: What they're for, how to update, how they work for adding fields, how to test, how they're deployed on production and locally
 * Composer: How we run it locally, on production and in CI, and why, and troubleshooting tips
 * Process-control
 * Deploymet: Steps for all service in the FR stack and config changes
 * Integrations: Adyen Checkout: Explanation of flow for different payment options (cc, cc with 3DS, rtbt), including pointers to codebases and methods responsible for each step, and UML interaction diagrams
 * DonationInterface: Overview of main classes and methods and their functions in the system
 * LandingCheck extension: where it's installed, how FR systems use it, link to documentation from other FR-Tech documentation.
 * List of public and non-public sites involved in FR systems, and which codebases are installed on each of them, and which servers host them.
 * DonationInterface: Config: Short summary of global config variables and per-processor config mechanism on wiki, with link to README for details; complete doc of all config variables in README, explanation of config in yml files in config folders in source code, explanation of which types of config should go in which locations.
 * Pipeline for selection of country, payment processor, method, recurring, and other variants, from banners or links in e-mails to Payments site.