Consolidate chapter page data into single file (#144)

## Summary

* Consolidate chapter page data into single file
* Header structure of the file stays the same, following the page count,
we now put a LUT offset
   * The page data is all then appended to this file
* Finally the LUT is appended to the end of the file, and the page count
is updated
* This will also significantly improve the duration of cache cleanup
which takes a while to scan the directory and cleanup content
* Remove page file version as it's all tied up into the section file now
* Bumped section file version to 7
* Moved section content into sub directory
* Updated docs

## Additional Context

* Benchmarks:
  * Generating 74 pages of content from a chapter in Jade Legacy took:
    * master: 6,229ms
    * this PR: 1,305ms
    * Speedup of 79%
  * Generating 207 pages of content from Livesuit book:
    * With progress bar UI updates:
      * master: 24,250ms
      * this PR: 8,063ms
      * Speedup of 67%
    * Without progress bar UI updates:
      * master: 13,055ms
      * this PR: 3,600ms
      * Speedup of 72%

lib/Epub/Epub/Page.cpp

@@ -3,20 +3,16 @@
 #include <HardwareSerial.h>
 #include <Serialization.h>
 
-namespace {
-constexpr uint8_t PAGE_FILE_VERSION = 3;
-}
-
 void PageLine::render(GfxRenderer& renderer, const int fontId, const int xOffset, const int yOffset) {
   block->render(renderer, fontId, xPos + xOffset, yPos + yOffset);
 }
 
-void PageLine::serialize(File& file) {
+bool PageLine::serialize(File& file) {
   serialization::writePod(file, xPos);
   serialization::writePod(file, yPos);
 
   // serialize TextBlock pointed to by PageLine
-  block->serialize(file);
+  return block->serialize(file);
 }
 
 std::unique_ptr<PageLine> PageLine::deserialize(File& file) {
@@ -35,27 +31,22 @@ void Page::render(GfxRenderer& renderer, const int fontId, const int xOffset, co
   }
 }
 
-void Page::serialize(File& file) const {
-  serialization::writePod(file, PAGE_FILE_VERSION);
-
+bool Page::serialize(File& file) const {
   const uint32_t count = elements.size();
   serialization::writePod(file, count);
 
   for (const auto& el : elements) {
     // Only PageLine exists currently
     serialization::writePod(file, static_cast<uint8_t>(TAG_PageLine));
-    el->serialize(file);
+    if (!el->serialize(file)) {
+      return false;
+    }
   }
+
+  return true;
 }
 
 std::unique_ptr<Page> Page::deserialize(File& file) {
-  uint8_t version;
-  serialization::readPod(file, version);
-  if (version != PAGE_FILE_VERSION) {
-    Serial.printf("[%lu] [PGE] Deserialization failed: Unknown version %u\n", millis(), version);
-    return nullptr;
-  }
-
   auto page = std::unique_ptr<Page>(new Page());
 
   uint32_t count;