Add string to documentation index

Author: mborland

doc/modules/ROOT/nav.adoc

@@ -47,6 +47,7 @@
 * xref:charconv.adoc[]
 * xref:stream.adoc[]
 * xref:numeric.adoc[]
+* xref:string.adoc[]
 * Benchmarks
 ** xref:u128_benchmarks.adoc[]
 *** xref:u128_benchmarks.adoc#u128_linux[Linux]

doc/modules/ROOT/pages/api_reference.adoc

@@ -244,6 +244,16 @@ Listed by analogous STL header.
 | Midpoint between two values
 |===
 
+[#api_string]
+=== xref:string.adoc[`<string>`]
+[cols="1,2", options="header"]
+|===
+| Function | Description
+
+| xref:string.adoc[`to_string`]
+| `std::string` conversion of base-10 values
+|===
+
 [#api_macros]
 == Macros
 

doc/modules/ROOT/pages/string.adoc

@@ -0,0 +1,34 @@
+////
+Copyright 2026 Matt Borland
+Distributed under the Boost Software License, Version 1.0.
+https://www.boost.org/LICENSE_1_0.txt
+////
+
+[#String]
+= `<string>`
+:idprefix: string_
+
+[source,c++]
+----
+#include <boost/int128/string.hpp>
+----
+
+The following are functions analogous to those found in `<string>` for builtin integer types.
+
+[source,c++]
+----
+#include <boost/int128/string.hpp>
+
+namespace boost {
+namespace int128 {
+
+inline std::string to_string(const int128_t& value);
+inline std::string to_string(const uint128_t& value);
+
+} // namespace int128
+} // namespace boost
+----
+
+Returns a `std::string` containing the decimal(base-10) representation of `value`.
+These functions may throw `std::bad_alloc` from the constructor of `std::string`.
+