More docs for Plane, Pi, etc. (#6850)

* Document the units of PI

Signed-off-by: Nick Cameron <nrc@ncameron.org>

* Add links between lang and std references

Signed-off-by: Nick Cameron <nrc@ncameron.org>

* Change signature of conversion functions

Signed-off-by: Nick Cameron <nrc@ncameron.org>

* Split foreign imports out of modules docs

Signed-off-by: Nick Cameron <nrc@ncameron.org>

* More docs for Plane

Signed-off-by: Nick Cameron <nrc@ncameron.org>

* Update docs/kcl-std/consts/std-math-PI.md

Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>

* Update rust/kcl-lib/std/math.kcl

Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>

---------

Signed-off-by: Nick Cameron <nrc@ncameron.org>
Co-authored-by: Jess Frazelle <jessfraz@users.noreply.github.com>
Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>

rust/kcl-lib/std/math.kcl

@@ -7,6 +7,14 @@ import Point2d from "std::types"
 
 /// The value of `pi`, Archimedes’ constant (π).
 ///
+/// `PI` is a number and is technically a ratio, so you might expect it to have type `number(_)`.
+/// However, `PI` is nearly always used for converting between different units - usually degrees to or
+/// from radians. Therefore, `PI` is treated a bit specially by KCL and always has unknown units. This
+/// means that if you use `PI`, you will need to give KCL some extra information about the units of numbers.
+/// Usually you should use type ascription on the result of calculations, e.g., `(2 * PI): number(rad)`.
+/// You might prefer to use `units::toRadians` or `units::toDegrees` to convert between angles with
+/// different units.
+///
 /// ```
 /// circumference = 70
 ///