Cleaned up Introduction

Rewrote the introduction chapter, in line with #138 (closed). Changes by section:

General

• Swapped Our Philosophy and Why a New OS?, to group related sections more closely
• Merged Why Rust? and Unsafes
• Removed The Redox community and Projects maintainers, since they become outdated quickly and are better suited to Gitlab

Welcome

• Removed authorship credits (they're better suited to the README, and they're outdated anyway)
• Added a note that the book is currently being (re)written
• Made capitalisation of Getting started link consistent with section title

What is Redox?

• Changed Linux hyperlink to point to the Linux Foundation homepage instead of Wikipedia (kept the POSIX one because their homepage isn't very informative)
• Added a TODO: update next to the supported hardware section
• Replaced the chapter breakdown with a TODO, pending a refactor

Our Philosophy

• Added a TODO: update next to the MIT X11 exceptions section
• Separated the implications of the microkernel on drivers out from the license properties section, and expanded on it a bit

Why a New OS?

• Fixed some awkward wording
• Removed the broken link to Why Free Software (did not link to Our Philosophy instead because we don't talk a lot about our license choice motivations there)
• Added link to Orbital repo

Redox vs. Other Operating Systems

• Renamed to "Comparing Redox with Other OSes"
• Changed "resources (will be explained later)" to "resources (TODO: link)", pending refactor
• Fixed some awkward wording
• Removed the exact line count of the kernel, replaced with TODO, for someone who knows the current number

Why Rust? / Unsafes

• Cleaned up some wording
• Added a note about source auditing