Design updates for #2015 numeric type literal syntax

[avicorp]

Edit numeric literal design for the literal type proposal and add reference where #2015 was mentioned.

Closes #2159

[chandlerc]

Thanks for the design updates! Adding @josh11b to help with review here as he helped review the original proposal.

[josh11b]

Please follow the instructions at https://github.com/carbon-language/carbon-lang/blob/trunk/docs/project/contribution_tools.md to set up pre-commit and prettier for auto-formatting.

[josh11b]

I talked with @zygoloid about how we should organize the docs, and he had the idea, which I like, that:

• eventually the docs/design/expressions/ section describes all kinds of expressions, which would include numeric type literals
• it would be reasonable to have a single document that covers all kinds of literals, especially if we don't have much to say about them, and only break out specific kinds of literals to their own documents if there are enough details to warrant that (more than 3 pages of text, say)

The path forward would be to move this content into docs/design/expressions/literals.md and link to it from docs/design/expressions/README.md; if literals.md gets too big we can think about splitting it up then.

[avicorp]

@josh11b, how can we continue with the PR?

[avicorp]

@zygoloid @josh11b Thanks for the Suggestions.

I am struggling to decide what scope should be defined in the overview section of literal expressions. In other languages, I have seen that all literal expressions are grouped together in the same document. However, here, we have split numeric floats and strings under Lexical Conventions. What do you think about what I wrote in the overview?