Update Help Desk, Foundations Team and Technical Writers' mission page to take into account decisions from Tails Summit 2017

Originally created by @intrigeri on #14519 (Redmine)

Help Desk will stop investigating individual hardware support issues and instead will ask Foundations Team to handle them (ideally providing stats about how many people are impacted).

This ticket is also about:

• main purpose, handling of hardware support issues, tech writing (non-)expectations
• “Spend more time gathering qualitative and quantitative user data” that compensates “Spend less time on handling individual support requests”.
• Foundations team mission acts as an interface between Help desk and Technical writing

Feature Branch: doc/14519-help-desk-foundations-team-and-tech-writers-mission

Related issues

• Related to #14458 (closed)
• Has duplicate #14517 (closed)
• Has duplicate #14513 (closed)