The "Configuration Language" section was becoming rather unweildy, both
by having a lot of pages and by some of the pages being quite large in
themselves.
This is a first step towards breaking things up a little more, starting
with two changes:
- The "Configuration Language" navigation is now split into two
sub-headings "Configuration Blocks" and "Syntax".
- Some of the information about sub-blocks of the "terraform" block are
now given their own pages, because their content is quite complex
in itself.
- "Version Constraints" is now a page in its own right, rather than this
content being duplicated in slightly different forms across multiple
contexts that make use of user-specified version constraints.