docs: various updates and text fixes in HTML for MD conversion

This PR contains various updates and textual fixes (incl. adding missing intro sentences) in preparation of the planned markdown conversion (see also OSGeo#4748).

For an easier review, the changes have been submitted in separate commits:

- [ ] **db**:
- [ ] **display**:
- [ ] **doc**:
- [ ] **general**:
- [ ] **gui**:
- [ ] **imagery**:
- [ ] **lib**:
- [ ] **misc**:
- [ ] **raster**:
- [ ] **raster3d**:
- [ ] **scripts**:
- [ ] **temporal**:
- [ ] **vector**:
- [ ] **visualization**:

lib/cairodriver/cairodriver.html

@@ -7,7 +7,7 @@ <h2>DESCRIPTION</h2>
 
 The Cairo driver generates PNG, BMP, PPM, PS, PDF or SVG images by
 GRASS display commands, using the
-<a href="http://www.cairographics.org/">Cairo graphics library</a>.
+<a href="https://www.cairographics.org/">Cairo graphics library</a>.
 The image format is selected from the extension of the output file.
 
 The Cairo driver is used for GRASS display commands by default if
@@ -44,7 +44,7 @@ <h3>Environment variables</h3>
   </li>
   <li><b>GRASS_RENDER_ANTIALIAS</b><br> can be <em>default</em>,
     <em>none</em>, <em>gray</em>, or <em>subpixel</em>, corresponding to
-    <a href="http://www.cairographics.org/manual/cairo-cairo-t.html#cairo-antialias-t">cairo_antialias_t</a>
+    <a href="https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-antialias-t">cairo_antialias_t</a>
   </li>
   <li><b>GRASS_RENDER_FILE=filename</b><br>
      the name and format of the resulting image file, default is

lib/init/variables.html

@@ -1,5 +1,11 @@
 <!-- meta page description: GRASS variables and environment variables -->
-There are two types of variables:
+
+A variable in scripting is a symbolic name that holds data which can be
+used and modified during script execution. Variables allow scripts to
+store and manipulate values dynamically, making them more flexible and
+reusable.
+
+In GRASS GIS, there are two types of variables:
 <ul>
 <li><a href="#setting-shell-environment-variables">shell
 environment</a> variables,</li>

lib/pngdriver/pngdriver.html

@@ -90,7 +90,7 @@ <h2>NOTES</h2>
 
 <p>Further PNG file processing (e.g. quantization to 1 bit for monochrome
 images) can be done with <tt>pnmquant</tt> of
-the <a href="http://netpbm.sourceforge.net/">netpbm</a> tools.
+the <a href="https://netpbm.sourceforge.net/">netpbm</a> tools.
 
 <h2>SEE ALSO</h2>
 

lib/vector/vectorascii.html

@@ -132,5 +132,6 @@ <h2>SEE ALSO</h2>
 <em>
   <a href="v.in.ascii.html">v.in.ascii</a>,
   <a href="v.out.ascii.html">v.out.ascii</a>,
-  <a href="v.edit.html">v.edit</a>
+  <a href="v.edit.html">v.edit</a>,
+  <a href="v.support.html">v.support</a>
 </em>