Improve formatting of developer notes

Summary:

Summary of changes:

 * Add a TOC to the page
 * Make tips and tricks section use h3 headings
 * Reformat and clarify some sections

Backport of core PR12759
https://github.com/bitcoin/bitcoin/pull/12759/files

Depends on D3961.

Test Plan:
Read it and try the TOC with a compatible rendering software (I used
grip from https://github.com/joeyespo/grip).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3945