dslx_reference improvements

I'm going to record some things I noticed that need improvement:

- [x] In the [Precedence ](https://google.github.io/xls/dslx_reference/#operator-precedence)table, mis-rendered `||` and `|`
- https://github.com/google/xls/pull/1292
- [x] In the Precedence table, `>>>` should not be there because it does not exist
- https://github.com/google/xls/pull/1292
- [x] concatenate operator is missing from [Precedence](https://google.github.io/xls/dslx_reference/#operator-precedence). I believe it's the same precedence as `+` and `-` (confirmed below)
- https://github.com/google/xls/pull/1298
- [ ] `..` the range expression is missing from Precedence. I believe it's lower than everything else (lower than boolean or)
- [ ] `/` and `%` are in the [table ](https://google.github.io/xls/dslx_reference/#operator-precedence)but not mentioned in [binary expressions](https://google.github.io/xls/dslx_reference/#binary-expressions). Both operators are worth describing their precise semantics (e.g. is `%` the [remainder or the modulo function](https://stackoverflow.com/questions/13683563/whats-the-difference-between-mod-and-remainder)?)
- [ ] `as` is never introduced as a [binary expression ](https://google.github.io/xls/dslx_reference/#binary-expressions)or operator. It looks like this [section](https://google.github.io/xls/dslx_reference/#numerical-conversions) is about `as`. But I think the current way things are organized and described could be improved. E.g., `as` is clearly a binary expression as it takes two arguments. Why not make it a subsection under Binary Expressions? This suggestion also applies comparison operators and concatenation operator. Anything that's in the precedence table should probably be grouped as a unary or binary expression/operator.
- [x] `(...)`, the grouping expression, is never is never mentioned as an expression, nor is its precedence mentioned (I assume it's the highest precedence?)
- https://github.com/google/xls/pull/1298
- [ ] `true` and `false` are never introduced as a [bit type](https://google.github.io/xls/dslx_reference/#bit-type)
- [ ] 'expression statements' are never documented. E.g. `Foo();`
- [ ] `let` expressions are not expressions. They are statements. In the past they were expressions but they aren't now. Update the wording and move to the statement section.