docs: add missing section closing comments in math/base/special READMEs

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---

Author: Planeshifter

lib/node_modules/@stdlib/math/base/special/cosd/README.md

@@ -26,6 +26,8 @@ limitations under the License.
 
 </section>
 
+<!-- /.intro -->
+
 <section class="usage">
 
 ## Usage

lib/node_modules/@stdlib/math/base/special/cosdf/README.md

@@ -26,6 +26,8 @@ limitations under the License.
 
 </section>
 
+<!-- /.intro -->
+
 <section class="usage">
 
 ## Usage

lib/node_modules/@stdlib/math/base/special/cot/README.md

@@ -26,6 +26,8 @@ limitations under the License.
 
 </section>
 
+<!-- /.intro -->
+
 <section class="usage">
 
 ## Usage

lib/node_modules/@stdlib/math/base/special/cotd/README.md

@@ -26,6 +26,8 @@ limitations under the License.
 
 </section>
 
+<!-- /.intro -->
+
 <section class="usage">
 
 ## Usage

lib/node_modules/@stdlib/math/base/special/cotdf/README.md

@@ -26,6 +26,8 @@ limitations under the License.
 
 </section>
 
+<!-- /.intro -->
+
 <section class="usage">
 
 ## Usage

lib/node_modules/@stdlib/math/base/special/cotf/README.md

@@ -26,6 +26,8 @@ limitations under the License.
 
 </section>
 
+<!-- /.intro -->
+
 <section class="usage">
 
 ## Usage

lib/node_modules/@stdlib/math/base/special/csc/README.md

@@ -26,6 +26,8 @@ limitations under the License.
 
 </section>
 
+<!-- /.intro -->
+
 <section class="usage">
 
 ## Usage

lib/node_modules/@stdlib/math/base/special/cscf/README.md

@@ -26,6 +26,8 @@ limitations under the License.
 
 </section>
 
+<!-- /.intro -->
+
 <section class="usage">
 
 ## Usage

lib/node_modules/@stdlib/math/base/special/logaddexp/README.md

@@ -28,6 +28,8 @@ Log-domain computations are commonly used to increase accuracy and avoid underfl
 
 </section>
 
+<!-- /.intro -->
+
 <section class="usage">
 
 ## Usage

lib/node_modules/@stdlib/math/base/special/sec/README.md

@@ -26,6 +26,8 @@ limitations under the License.
 
 </section>
 
+<!-- /.intro -->
+
 <section class="usage">
 
 ## Usage