Update docs for v0.5.0

[edwardalee]

I have started a branch release-5 in which we should document changes that will become part of the 0.5.0 release. Off the top of my head, the following is a to-do list (this should be updated using the release notes). The checked items are either in this branch already or in main already. Please add to this list.

• New parameter and state initialization syntax
• New preamble architecture
• math.h is included automatically in the C target
• single-threaded and workers options in target and on the command line.
• change indentation to two spaces instead of four.
• add reset keyword for state variable.
• add reset trigger for reactions.
• document generic reactors in C
• reaction declarations
• fix Cpp description in Expressions.md once this bug is fixed.
• fix Python description in Expressions.md once this bug is fixed (bug is not fixed. documented old syntax.)

[edwardalee]

I have completed a pass on the docs. This is ready to review. When we merge it, I think the handbook will be up-to-date with release 0.5. Please merge.

[lhstrh]

@edwardalee, I'm a little puzzled by this FIXME...

[edwardalee]

I didn't add this comment, but I did notice that the other targets use a timeout of 0 for this test.

[lhstrh]

Thanks for working on this so tirelessly, @edwardalee!