intro manual pages: standardize first subsection

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@59810 15284696-431f-4ddb-bdfa-cd5b030d7da7

doc/projectionintro.html

@@ -1,5 +1,5 @@
 <!-- meta page description: Projections and spatial transformations in GRASS GIS-->
-<h3>Introduction</h3>
+<h3>Projection management in general</h3>
 
 A GRASS location is referenced with a single projection and coordinate system
 (or unreferenced as XY location). When creating a new location from an existing

imagery/imageryintro.html

@@ -1,6 +1,6 @@
 <!-- meta page description: Image processing in GRASS GIS -->
 <!-- meta page index: imagery -->
-<h3>General introduction</h3>
+<h3>Image processing in general</h3>
 
 <b>Digital numbers and physical values (reflection/radiance-at-sensor):</b>
 <p>

raster3d/raster3dintro.html

@@ -1,5 +1,7 @@
 <!-- meta page description: 3D raster data (voxel) processing in GRASS GIS -->
 <!-- meta page index: raster3D -->
+<h3>3D Raster maps (voxels) in general</h3>
+
 GRASS GIS is one of the few GIS software packages with volume data support.
 Here data are stored as a 3D raster with a unit volume called a voxel
 (volume pixel). Voxels are designed to support representations of

temporal/temporalintro.html

@@ -1,5 +1,6 @@
 <!-- meta page description: Temporal data processing in GRASS GIS -->
 <!-- meta page index: temporal -->
+<h3>The temporal GIS framework in general</h3>
 
 The temporal GIS framework in GRASS introduces three new datatypes that
 are designed to handle time series data: