[std] Create new top-level Clause [text], following [strings]

Part of the C++26 clause restructuring (#5315).

Author: tkoeppe

source/std.tex

@@ -134,6 +134,7 @@
 \include{ranges}
 \include{algorithms}
 \include{strings}
+\include{text}
 \include{numerics}
 \include{time}
 \include{locales}

source/text.tex

@@ -0,0 +1,12 @@
+%!TEX root = std.tex
+
+\rSec0[text]{Text processing library}
+
+\rSec1[text.general]{General}
+
+This Clause describes components for dealing with text.
+These components are summarized in \tref{text.summary}.
+
+\begin{libsumtab}{Text library summary}{text.summary}
+\ref{text}               & (placeholder)         & (placeholder)    \\
+\end{libsumtab}