Reorganize Learning Path for new learners

[HuskyNator]

In order to introduce new people to the language & hold their interest, I feel the page order of the language tour should be changed, quite a bit.

When comparing the order of pages, the tour tries to jump into more complex topics before describing the basics. Additionally, some sections seem misplaced (see bottom notes regarding "Ranges" and "Documentation")
This should make the tour more understandable to people less experienced in languages like C & provide a more organized approach.

Current "Basics" order:

1. Imports/Modules
2. Basic types
3. Memory
4. Mutability
5. Control flow
6. Functions
7. Structs
8. Arrays
9. Slices
10. Alias & Strings
11. Loops
12. Foreach
13. Ranges
14. Associative Arrays
15. Classes
16. Interfaces
17. Templates
18. Delegates
19. Exceptions
20. Further Reading

My Suggestion:

1. Hello World (explicitly as one section, at the start, see note below)
2. Basic types
3. Functions
4. Comments (/"Documentation", see note below)
5. Control Flow (merge "Loops" into this Merge Loops into Control Flow #373 & Merges Loops & Control Flow #374 )
6. Structs
7. Enums (explicit addition)
8. Arrays
9. Foreach
10. Memory
11. Mutability
12. Alias & Strings
13. Associative Arrays
14. Slices
15. Classes
16. Interfaces
17. Templates
18. Delegates
19. Exceptions
20. Imports and Modules
21. Further Reading

Important notes about this change: (may need to be split into separate issues)

• Adds explicit "Hello World" section (Move out of Import example & move rest of this section to the end of the basics track. Can still refer to this section for the examples import std.stdio: writeln; statement)
• Adds explicit "Enums" section.
• Moves "Ranges" from "Basics" to "D's Gems" (immediately before "Range Algorithms")
• Moves "Documentation" from "D's Gems" to "Basics" and complete section with more on comments in general.

[HuskyNator]

Taking a bit of inspiration from other books & guides like; might be useful too:

• Programming in D
• The Rust Programming Language Book

[pbackus]

• The C Programming Language
• How to Design Programs