Merge pull request #959 from boriel-basic/docs

doc: add PI documentation

Author: boriel

docs/pi.md

@@ -0,0 +1,31 @@
+# PI
+
+## Syntax
+
+```
+PI
+```
+
+
+## Description
+
+PI is a math _constant_, roughly equivalent to **3.141592654...**,
+and it is of type `Float`
+
+## Example
+
+```
+REM Prints PI Value
+PRINT "PI = "; PI
+```
+Which will print a _rounded value_ of `3.1415927`
+
+
+## Remarks
+
+*  This constant is 100% Sinclair BASIC Compatible.
+
+## See also
+* [COS](cos.md) and [ACS](acs.md)
+* [SIN](sin.md) and [ASN](asn.md)
+* [TAN](tan.md) and [ATN](atn.md)