doc: move style guide to findable location

Move the style guide to doc/README.md so people might find it. The
current location is easily overlooked, as is evidenced by
nodejs#41025

doc/guides/doc-style-guide.md → doc/README.md

@@ -109,7 +109,7 @@ For topics not covered here, refer to the [Microsoft Writing Style Guide][].
 [Use sentence-style capitalization for headings]: https://docs.microsoft.com/en-us/style-guide/scannable-content/headings#formatting-headings
 [Use serial commas]: https://docs.microsoft.com/en-us/style-guide/punctuation/commas
 [`remark-preset-lint-node`]: https://github.com/nodejs/remark-preset-lint-node
-[doctools README]: ../../tools/doc/README.md
+[doctools README]: ../tools/doc/README.md
 [info string]: https://github.github.com/gfm/#info-string
 [language]: https://github.com/highlightjs/highlight.js/blob/HEAD/SUPPORTED_LANGUAGES.md
 [plugin]: https://editorconfig.org/#download

doc/guides/contributing/pull-requests.md

@@ -121,7 +121,7 @@ If you are modifying code, please be sure to run `make lint` (or
 code style guide.
 
 Any documentation you write (including code comments and API documentation)
-should follow the [Style Guide](../doc-style-guide.md). Code samples
+should follow the [Style Guide](../../README.md). Code samples
 included in the API docs will also be checked when running `make lint` (or
 `vcbuild.bat lint` on Windows). If you are adding to or deprecating an API,
 add or change the appropriate YAML documentation. Use `REPLACEME` for the