RFR: Split up monolithic markdown

[Magnus Ihse Bursie]

The original Developer's Guide was split into one html page per chapter. This was (rightly so) combined into a single html page previously by Jesper.

However, this also means that we got a single, monolithic markdown file to edit. This makes the file unnecessarily hard to navigate and edit.

This patch will instead split up the original index.md file into separate markdown documents, one per chapter. At build time, they are recombined into a single index.md file again (exactly identical with the one I started with).

This patch depends on https://github.com/openjdk/guide/pull/46.

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Commit messages:
 - Split up index.md into one file per chapter
 - Additional improvements for reusability and debuggability
 - Improve and modernize the makefile

Changes: https://git.openjdk.java.net/guide/pull/47/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=47&range=00
  Stats: 3697 lines in 14 files changed: 1869 ins; 1815 del; 13 mod
  Patch: https://git.openjdk.java.net/guide/pull/47.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/47/head:pull/47

PR: https://git.openjdk.java.net/guide/pull/47