docs: categorize Language Definition functions (expr-lang#452)

* docs: categorize Language Definition functions

- multi-type / misc functions
- Type Conversion functions
- String functions
- Date functions
- Number functions
- Array functions
- Map functions

- also consistently use `str`, `array`, `n`, `v` for the variable name when referring to a specific type
  - `array` was used consistently for older functions, but not some newer ones
  - `str` added to match `array`
  - `n` was sometimes used for numbers or ints, but not always
  - `v` is for multi-type "values"

Signed-off-by: Anton Gilgur <[email protected]>

* review feedback -- categories to h2

- categories to h2, individual functions to h3 per review feedback

- rename "Built-in Functions" -> "Miscellaneous Functions" as this is now only around the uncategorized functions
  - move this to the bottom of the function list as well so that "miscellaneous" is not first

- combine "Date Manipulation" with "Date Functions"

Signed-off-by: Anton Gilgur <[email protected]>

* move type functions to second-to-last before misc functions, per code review

Signed-off-by: Anton Gilgur <[email protected]>

* move number to after string per code review

Signed-off-by: Anton Gilgur <[email protected]>

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Signed-off-by: Anton Gilgur <[email protected]>

Author: agilgur5