docs: Add SVG diagram style guide and PNG generation instructions

New README.md in docs/manual/images/ documents the font, color, and
layout conventions used for flowchart SVGs, includes the full SVG
boilerplate template, and provides rsvg-convert install/usage
instructions for macOS (brew), Fedora/RHEL (dnf), and Debian/Ubuntu
(apt-get).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1935414 13f79535-47bb-0310-9956-ffa450edef68

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+# SVG Diagram Style Guide
+
+This documents the style conventions used for flowchart/diagram SVGs
+in this directory, so that new diagrams are visually consistent.
+
+## Fonts
+
+- **Primary:** Arial, Helvetica, sans-serif
+- **Title:** 12px, bold
+- **Phase/section labels:** 11px, bold, fill `#444`
+- **Body text:** 10px, fill `#333`
+- **Edge labels (Yes/No):** 9px, fill `#555`
+- **Annotations/italic notes:** 8px, fill `#666`, italic
+
+## Colors
+
+| Element | Fill | Stroke | Notes |
+|---------|------|--------|-------|
+| Process box (action) | `#e8e8e8` | `#333` | `rx:4 ry:4` rounded corners |
+| Decision diamond | `#fff8dc` (cornsilk) | `#333` | |
+| Terminal (start/done) | `#d4edda` (light green) | `#333` | `rx:12 ry:12` pill shape |
+| Redirect / error | `#f8d7da` (light red) | `#333` or `#b33` | `rx:12 ry:12` pill shape |
+| Phase box (grouping) | `#fafafa` | `#888` | `rx:8 ry:8`, contains related steps |
+| Warning box | `#f8d7da` | `#b33` | `rx:4 ry:4`, stroke-width 1.5 |
+| Lines/arrows | — | `#333` | stroke-width 1.2 |
+| Dashed lines (loops) | — | `#333` | stroke-dasharray: 5,4 |
+
+## Arrowhead Marker
+
+```xml
+<marker id="arrowhead" markerWidth="10" markerHeight="7" refX="9" refY="3.5" orient="auto">
+  <polygon points="0 0, 10 3.5, 0 7" fill="#333"/>
+</marker>
+```
+
+## SVG Boilerplate
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 {W} {H}" width="{W}" height="{H}">
+  <defs>
+    <marker id="arrowhead" markerWidth="10" markerHeight="7" refX="9" refY="3.5" orient="auto">
+      <polygon points="0 0, 10 3.5, 0 7" fill="#333"/>
+    </marker>
+    <style>
+      text { font-family: Arial, Helvetica, sans-serif; font-size: 10px; fill: #333; }
+      .title { font-size: 12px; font-weight: bold; }
+      .phase { font-size: 11px; font-weight: bold; fill: #444; }
+      .label { font-size: 9px; fill: #555; }
+      .small { font-size: 8px; fill: #666; font-style: italic; }
+      rect.process { fill: #e8e8e8; stroke: #333; stroke-width: 1.2; rx: 4; ry: 4; }
+      polygon.decision { fill: #fff8dc; stroke: #333; stroke-width: 1.2; }
+      rect.terminal { fill: #d4edda; stroke: #333; stroke-width: 1.2; rx: 12; ry: 12; }
+      rect.redirect { fill: #f8d7da; stroke: #333; stroke-width: 1.2; rx: 12; ry: 12; }
+      rect.phase-box { fill: #fafafa; stroke: #888; stroke-width: 1.2; rx: 8; ry: 8; }
+      line, path { stroke: #333; stroke-width: 1.2; fill: none; marker-end: url(#arrowhead); }
+      .dashed { stroke-dasharray: 5,4; }
+    </style>
+  </defs>
+  <!-- diagram content here -->
+</svg>
+```
+
+## Generating PNGs from SVGs
+
+Use `rsvg-convert` (from `librsvg`):
+
+```bash
+# Install (macOS):
+brew install librsvg
+
+# Install (Fedora/RHEL/CentOS):
+dnf install librsvg2-tools
+
+# Install (Debian/Ubuntu):
+apt-get install librsvg2-bin
+
+# Convert at 1x (matching SVG viewBox dimensions):
+rsvg-convert -o rewrite_l_flag_looping.png rewrite_l_flag_looping.svg
+
+# Or specify explicit dimensions:
+rsvg-convert -w 520 -h 720 -o rewrite_l_flag_looping.png rewrite_l_flag_looping.svg
+```
+
+Existing PNGs in this directory are at 1x scale (matching their SVG
+viewBox width/height). Keep PNGs at 1x for consistency with the rest
+of the documentation build.
+
+## General Conventions
+
+- Diamonds for decisions, rounded rectangles for actions, pill shapes
+  for start/end terminals.
+- Yes/No labels on decision branches (9px, gray).
+- Phase boxes group related steps that occur in the same processing context.
+- Dashed lines indicate loop-back paths or optional flows.
+- Titles centered at the top of the SVG.
+- Typical viewBox widths: 440–650px. Heights: 360–750px.