doc: enable first-heading-level remark-lint rule

This makes sure that the first heading in all markdown files is h1.

Only three files needed to be changed to conform:
 1. BUILDING.md
 2. CODE_OF_CONDUCT.md
 3. doc/onboarding-extras.md

Also, .remarkrc is updated to include the `first-heading-level: 1`
rule in order to catch similar issues in the future.

PR-URL: nodejs#8716
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: ChALkeR

.remarkrc

@@ -13,7 +13,7 @@
       "file-extension": "md",
       "final-definition": true,
       "final-newline": true,
-      "first-heading-level": false,
+      "first-heading-level": 1,
       "hard-break-spaces": true,
       "heading-increment": false,
       "heading-style": "atx",

BUILDING.md

@@ -1,4 +1,4 @@
-## Building Node.js
+# Building Node.js
 
 Depending on what platform or features you require, the build process may
 differ slightly. After you've successfully built a binary, running the

CODE_OF_CONDUCT.md

@@ -1,6 +1,6 @@
-## Code of Conduct
+# Code of Conduct
 
-### Conduct
+## Conduct
 
 * We are committed to providing a friendly, safe and welcoming
   environment for all, regardless of level of experience, gender
@@ -38,17 +38,17 @@
   documentation. There is no need to address persons when explaining
   code (e.g. "When the developer").
 
-### Contact
+## Contact
 Instances of abusive, harassing, or otherwise unacceptable behavior may be
 reported by:
 
 * Emailing [report@nodejs.org](mailto:report@nodejs.org) (this will email all TSC members)
 * Contacting [individual TSC members](https://nodejs.org/en/foundation/tsc/#current-members-of-the-technical-steering-committee).
 
-### Moderation
+## Moderation
 See the TSC's [moderation policy](https://github.com/nodejs/TSC/blob/master/Moderation-Policy.md) for details about moderation.
 
-### Attribution
+## Attribution
 
 This Code of Conduct is adapted from [Rust's wonderful
 CoC](http://www.rust-lang.org/conduct.html) as well as the

doc/onboarding-extras.md

@@ -1,3 +1,5 @@
+# Additional Onboarding Information
+
 ## Who to CC in issues
 
 | subsystem | maintainers |