Merge branch 'docs/string-reference' into 'dev'

Add string reference page to docs

See merge request ligolang/ligo!392

gitlab-pages/docs/reference/string.md

@@ -0,0 +1,79 @@
+---
+id: string-reference
+title: String
+---
+
+## String.size(s: string) : nat
+
+Get the size of a string. [Michelson only supports ASCII strings](http://tezos.gitlab.io/whitedoc/michelson.html#constants) 
+so for now you can assume that each character takes one byte of storage.
+
+<!--DOCUSAURUS_CODE_TABS-->
+
+<!--PascaLIGO-->
+```pascaligo
+function string_size (const s: string) : nat is size(s)
+```
+
+<!--CameLIGO-->
+```cameligo
+let size_op (s: string) : nat = String.size s
+```
+
+<!--ReasonLIGO-->
+```reasonligo
+let size_op = (s: string): nat => String.size(s);
+```
+
+<!--END_DOCUSAURUS_CODE_TABS-->
+
+## String.length(s: string) : nat
+
+Alias for `String.size`.
+
+## String.slice(pos1: nat, pos2: nat, s: string) : string
+
+Get the substring of `s` between `pos1` inclusive and `pos2` inclusive. For example
+the string "tata" given to the function below would return "at".
+
+<!--DOCUSAURUS_CODE_TABS-->
+<!--PascaLIGO-->
+```pascaligo
+function slice_op (const s : string) : string is string_slice(1n , 2n , s)
+```
+<!--CameLIGO-->
+```cameligo
+let slice_op (s: string) : string = String.slice 1n 2n s
+```
+<!--ReasonLIGO-->
+```reasonligo
+let slice_op = (s: string): string => String.slice(1n, 2n, s);
+```
+<!--END_DOCUSAURUS_CODE_TABS-->
+
+## String.sub(pos1: nat, pos2: nat, s: string) : string
+
+Alias for `String.slice`.
+
+## String.concat(s1: string, s2: string) : string
+
+Concatenate two strings and return the result.
+
+<!--DOCUSAURUS_CODE_TABS-->
+
+<!--PascaLIGO-->
+```pascaligo
+function concat_op (const s : string) : string is s ^ "toto"
+```
+
+<!--CameLIGO-->
+```cameligo
+let concat_syntax (s: string) = s ^ "test_literal"
+```
+
+<!--ReasonLIGO-->
+```reasonligo
+let concat_syntax = (s: string) => s ++ "test_literal";
+```
+
+<!--END_DOCUSAURUS_CODE_TABS-->

src/test/md_file_tests.ml

@@ -122,6 +122,7 @@ let md_files = [
   "/gitlab-pages/docs/advanced/timestamps-addresses.md";
   "/gitlab-pages/docs/api/cli-commands.md";
   "/gitlab-pages/docs/api/cheat-sheet.md";
+  "/gitlab-pages/docs/reference/string.md";
 ]
 
 let md_root = "../../gitlab-pages/docs/language-basics/"