doc/markdown file organization lacks coherence

[sam-github]

Over time, the docs have spread across many directory levels. I suggest they be moved around a bit to simplify the arrangement for consistency and discoverability.

Current state, edited to remove docs that are clearly in the right place (doc/changelogs, doc/api), with notes:

.
├── BUILDING.md
├── CHANGELOG.md
├── CODE_OF_CONDUCT.md
├── CONTRIBUTING.md
├── GOVERNANCE.md
├── README.md
└── SECURITY.md
> Above are pretty traditional for project root dirs.
├── COLLABORATOR_GUIDE.md
> Its a guide, move to doc/guides/. CONTRIBUTING is the top-level for people just starting
├── CPP_STYLE_GUIDE.md
> Its a guide, move to doc/guides/.
├── benchmark
│   ├── README.md
│   └── writing-and-running-benchmarks.md
> Its a guide, move to doc/guides/writing-benchmarks, like doc/guides/writing-tests.md
├── doc
│   ├── guides
│   │   ├── adding-new-napi-api.md
│   │   ├── backporting-to-release-lines.md
│   │   ├── building-node-with-ninja.md
│   │   ├── contributing
│   │   │   ├── coc.md
│   │   │   ├── issues.md
│   │   │   └── pull-requests.md
> Perhaps there is a functional reason these three are here? Otherwise, coc.md seems like it should be CODE_OF_CONDUCT.md
│   │   ├── cve_management_process.md
│   │   ├── diagnostic-tooling-support-tiers.md
│   │   ├── internal
│   │   │   └── readme.md
> Not clear why this is here on its lonesome, move to doc/guides/internal-api.md? or doc/api/internal.md?
│   │   ├── maintaining-icu.md
│   │   ├── maintaining-npm.md
│   │   ├── maintaining-the-build-files.md
│   │   ├── maintaining-V8.md
│   │   ├── node-postmortem-support.md
│   │   ├── security_release_process.md
> Move to security-release-process.md
│   │   ├── updating-root-certs.md
> Move to maintaining-root-certs.md
│   │   ├── using-internal-errors.md
│   │   ├── using-symbols.md
│   │   └── writing-tests.md
.doc+
│   ├── offboarding.md
│   ├── onboarding-extras.md
│   ├── onboarding.md
│   ├── releases.md
> Move all 4 above to guides/? releases.md is here, security releases are in guides/, this is odd.
│   └── STYLE_GUIDE.md
> Move to guides/documentation-style.md?

Also: we have randomly use both README.md and readme.md as file names, and should probably just use the uppercase version:

./benchmark/README.md
./README.md
./test/wasi/README.md
./test/fixtures/0-dns/README.md
./test/fixtures/wpt/url/README.md
./test/fixtures/wpt/resources/webidl2/README.md
./test/fixtures/wpt/resources/readme.md
./test/fixtures/wpt/README.md
./test/README.md
./test/wpt/README.md
./test/common/README.md
./doc/guides/internal/readme.md
./lib/internal/readme.md
./src/README.md

[richardlau]

│   └── STYLE_GUIDE.md
> Move to guides/documentation-style.md?

and merge with tools/doc/README.md?

[DavenportEmma]

I'd like to take this issue, if that's ok

[sam-github]

Go for it!