RFR: Split up the monolithic index.md, take two [v2]

[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. ;-)

Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:

  If the TOC does not match reality, fail with a build error

-------------

Changes:
  - all: https://git.openjdk.java.net/guide/pull/82/files
  - new: https://git.openjdk.java.net/guide/pull/82/files/6e63ebe1..585911cb

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=guide&pr=82&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=guide&pr=82&range=00-01

  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 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