docs: Put render nodes in their own sections

docs/reference/gsk/gsk4-sections.txt

@@ -98,19 +98,22 @@ gsk_render_node_write_to_file
 GskScalingFilter
 gsk_render_node_get_bounds
 
-<SUBSECTION Nodes>
+<SUBSECTION>
 gsk_color_node_new
 gsk_color_node_get_color
 gsk_texture_node_new
 gsk_texture_node_get_texture
+<SUBSECTION>
 GskColorStop
 gsk_linear_gradient_node_new
+gsk_repeating_linear_gradient_node_new
 gsk_linear_gradient_node_get_start
 gsk_linear_gradient_node_get_end
 gsk_linear_gradient_node_get_n_color_stops
 gsk_linear_gradient_node_get_color_stops
-gsk_repeating_linear_gradient_node_new
+<SUBSECTION>
 gsk_radial_gradient_node_new
+gsk_repeating_radial_gradient_node_new
 gsk_radial_gradient_node_get_n_color_stops
 gsk_radial_gradient_node_get_color_stops
 gsk_radial_gradient_node_get_start
@@ -118,16 +121,18 @@ gsk_radial_gradient_node_get_end
 gsk_radial_gradient_node_get_hradius
 gsk_radial_gradient_node_get_vradius
 gsk_radial_gradient_node_get_center
-gsk_repeating_radial_gradient_node_new
+<SUBSECTION>
 gsk_conic_gradient_node_new
 gsk_conic_gradient_node_get_n_color_stops
 gsk_conic_gradient_node_get_color_stops
 gsk_conic_gradient_node_get_center
 gsk_conic_gradient_node_get_rotation
+<SUBSECTION>
 gsk_border_node_new
 gsk_border_node_get_outline
 gsk_border_node_get_widths
 gsk_border_node_get_colors
+<SUBSECTION>
 gsk_inset_shadow_node_new
 gsk_inset_shadow_node_get_outline
 gsk_inset_shadow_node_get_color
@@ -135,6 +140,7 @@ gsk_inset_shadow_node_get_dx
 gsk_inset_shadow_node_get_dy
 gsk_inset_shadow_node_get_spread
 gsk_inset_shadow_node_get_blur_radius
+<SUBSECTION>
 gsk_outset_shadow_node_new
 gsk_outset_shadow_node_get_outline
 gsk_outset_shadow_node_get_color
@@ -142,45 +148,57 @@ gsk_outset_shadow_node_get_dx
 gsk_outset_shadow_node_get_dy
 gsk_outset_shadow_node_get_spread
 gsk_outset_shadow_node_get_blur_radius
+<SUBSECTION>
 gsk_cairo_node_new
 gsk_cairo_node_get_draw_context
 gsk_cairo_node_get_surface
+<SUBSECTION>
 gsk_container_node_new
 gsk_container_node_get_n_children
 gsk_container_node_get_child
+<SUBSECTION>
 gsk_transform_node_new
 gsk_transform_node_get_child
 gsk_transform_node_get_transform
+<SUBSECTION>
 gsk_opacity_node_new
 gsk_opacity_node_get_child
 gsk_opacity_node_get_opacity
+<SUBSECTION>
 gsk_color_matrix_node_new
 gsk_color_matrix_node_get_child
 gsk_color_matrix_node_get_color_matrix
 gsk_color_matrix_node_get_color_offset
+<SUBSECTION>
 gsk_repeat_node_new
 gsk_repeat_node_get_child
 gsk_repeat_node_get_child_bounds
+<SUBSECTION>
 gsk_clip_node_new
 gsk_clip_node_get_child
 gsk_clip_node_get_clip
+<SUBSECTION>
 gsk_rounded_clip_node_new
 gsk_rounded_clip_node_get_child
 gsk_rounded_clip_node_get_clip
+<SUBSECTION>
 GskShadow
 gsk_shadow_node_new
 gsk_shadow_node_get_shadow
 gsk_shadow_node_get_n_shadows
 gsk_shadow_node_get_child
+<SUBSECTION>
 GskBlendMode
 gsk_blend_node_new
 gsk_blend_node_get_bottom_child
 gsk_blend_node_get_top_child
 gsk_blend_node_get_blend_mode
+<SUBSECTION>
 gsk_cross_fade_node_new
 gsk_cross_fade_node_get_start_child
 gsk_cross_fade_node_get_end_child
 gsk_cross_fade_node_get_progress
+<SUBSECTION>
 gsk_text_node_new
 gsk_text_node_get_font
 gsk_text_node_get_glyphs
@@ -188,12 +206,15 @@ gsk_text_node_get_color
 gsk_text_node_has_color_glyphs
 gsk_text_node_get_num_glyphs
 gsk_text_node_get_offset
+<SUBSECTION>
 gsk_blur_node_new
 gsk_blur_node_get_child
 gsk_blur_node_get_radius
+<SUBSECTION>
 gsk_debug_node_new
 gsk_debug_node_get_child
 gsk_debug_node_get_message
+<SUBSECTION>
 gsk_gl_shader_node_new
 gsk_gl_shader_node_get_n_children
 gsk_gl_shader_node_get_child