Update shell.md

Many tweaks to the Redox Book

The changes include:
- Editing for flow and understandability.
- Unifying how links are included in the source files.
- Adding an `About this Book` page to the introduction.
- Adding a few examples or new pieces of information I felt were
  lacking when I read the book.
- Unifying 'user space' and 'kernel space' spelling.
- Added myself to the authors list in `overview/welcome.md`.

update preparing_the_build.md & compiling_redox.md

Fix minor spelling and grammatical issues

Minor edits to first 2 chapters

Update what_is_redox.md

Fix typo

Go into more detail and reason about micro kernels

Changes to try and improve public relations

Conflicts:
	src/overview/developers.md

Add some links to developers.md file

Expand "Everything is an URL" section

fixed bad link

link did not link to wikipedia article. moved link to best guess of intended file

Add text to "Disadvantages of microkernels"