RFR: Split up the monolithic index.md, take two

[Magnus Ihse Bursie]

Here's a resurrection of my old code to split up the monolithic index.md.

I have fully recreated the per-chapter files based on the current content of index.md. I have removed the number prefix and instead the order of the files are determined by `src/toc.conf`.

By my quick inspection, the generated index.html looks identical to the one before.

I found some inconsistencies when doing this. For instance most, but not all, chapters end with a 

::: {.box}
[To the top](#){.boxheader}
:::

footer. A reasonable follow-up is to move these per-chapter footers out of the actual markdown, and inject them when combining the file instead. This means less redundancy, and no more inconsistencies. ;-)

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Commit messages:
 - Code conventions mistakenly slipped into Mailing lists as well
 - Add build support for combining split-up markdown files
 - Split index.md into per-chapter files

Changes: https://git.openjdk.java.net/guide/pull/82/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=82&range=00
  Stats: 4808 lines in 22 files changed: 2386 ins; 2418 del; 4 mod
  Patch: https://git.openjdk.java.net/guide/pull/82.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/82/head:pull/82

PR: https://git.openjdk.java.net/guide/pull/82