Make secrets.h more immune to changes from secrets.example.h

[robertlipe]

When new fields are added in the example file, add a test in the file that uses the real file to be sure the fields are present. This is to solve the case where a developer has a checked-out, working tree with a working secrets.h, syncs to head or another branch that adds fields in secrets, and then incurs weird compilation errors when those new fields aren't found.

This change leads the developer to the correct file and the doc describing it for a better experience.

Description

As discussed in #572.

Contributing requirements

• [x] I read the contribution guidelines in CONTRIBUTING.md.
• [x] I understand the BlinkenPerBit metric, and maximized it in this PR.
• [x] I selected main as the target branch.
• [x] All code herein is subjected to the license terms in COPYING.txt.