Merge #12759: [Docs] Improve formatting of developer notes

0bd2ec5 Improve formatting of developer notes (Evan Klitzke)

Pull request description:

  The developer notes file has gotten pretty large and unwieldly. This reorganizes some content, most notably by adding a TOC to the page. Compare how the page looks in [my branch](https://github.com/eklitzke/bitcoin/blob/developer-notes/doc/developer-notes.md) vs [master](https://github.com/eklitzke/bitcoin/blob/master/doc/developer-notes.md).

  I know there's long-term interest in moving a lot of this content to the bitcoin-core/docs repo on GitHub, but this makes things better now.

  The TOC format here is a semi-standard extension to Markdown files that you may have seen on GitHub pages in other projects. The `<!-- markdown-toc -->` comments used by these tools to know where the TOC starts/ends. The following tools all understand this format:

   * [Sphinx](http://www.sphinx-doc.org/en/master/)
   * [Pandoc](https://pandoc.org/)
   * [markdown-toc.el](https://github.com/ardumont/markdown-toc) (what I used)
   * various plugins for [vim](https://github.com/mzlogin/vim-markdown-toc) and other editors

  I used this TOC extension at a previous job, and my observation is that it's fine if people just edit the TOC manually. It's just text and it's not a huge deal if it gets a little out of sync. It can also be regenerated at any time by anyone with any of these tools.

Tree-SHA512: 298d1605ea5e8bfc0f75e70570c23ebd6891e4ffcdedd24fefadc23edd6e4b96509d8d102209868468a1b3ddbe2c3b8462698cdda8b9421348b5bc6f7b8d0cb8

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