TUTORIALS: Reorganise tutorials and add inline solutions

[bbrcknl]

The updates in this PR were written in conjunction with those in https://github.com/seL4/docs/pull/231

The aim of both PRs is:

To arrange the tutorial material on the docsite such that it is clear and easy to follow. Inspired by the Rust book https://doc.rust-lang.org/book/, the user can see the chapters in the index on the left, and go straight to the sections that they need.

Specifically, the updates on the docsite provide:

• a streamlined guide to completing the tutorials
• a streamlined "setting up your machine" page
• a how-to page with solutions to tutorial questions
• more organised landing page

The tutorials have been slightly rewritten to:

• correct minor errors
• provide inline solutions for readers wishing to understand the material and/or get quick solutions

Remaining issues:

• camkes-timer part 2 fails to compile (ninja) with current solutions - need someone with know-how to check what's going on
• I didn't test camkes-vm-linux; wasn't sure whether to spend a lot of time investigating KVM alternative for Mac
• the tutorial camkes-vm-crossvm fails to initialise - need someone with know-how to check what's going on

A note on using the rewritten docsite (from https://github.com/seL4/docs/pull/231): While the docsite in this PR has the new tutorial layout, the Makefile clones the old tutorials from https://github.com/seL4/sel4-tutorials.

You’ll need to integrate the updated tutorials (from this PR) in the Makefile from https://github.com/seL4/docs/pull/231 I can’t offer code for this because I’m a Makefile noob. I tested the new tutorials by having the Makefile call in and manipulate tutorial files from local directories.

[bbrcknl]

The links in this PR fail because they refer to the new docsite layout, see https://github.com/seL4/docs/pull/231

[bbrcknl]

@Indanz Minor fixes made to hello-world and mcs as per your suggestions. Original copyright restore for tute files.

[bbrcknl]

Dynamic libraries filenames need changing. It's on my to-dos for Friday.

[lsf37]

Squashed the commits as discussed on MM. Have a look if you're happy with the commit message.