[Chore] Update .npmignore to ignore .github and docs folders

[SergeAstapov]

Currently, running npm pack --dry-run --json | grep "github\|docs" returns long list with lots of files in /docs folder and everything under .github.

Tarball Details:

npm notice name:          ember-cli                               
npm notice version:       4.0.0-beta.2                            
npm notice filename:      ember-cli-4.0.0-beta.2.tgz              
npm notice package size:  531.9 kB                                
npm notice unpacked size: 2.8 MB

After the change, running npm pack --dry-run --json | grep "github\|docs" returns:

"path": "docs/build/data.json",
"path": ".github/workflows/ci.yml",
"path": "blueprints/addon/files/.github/workflows/ci.yml",
"path": "blueprints/app/files/.github/workflows/ci.yml",

With Tarball Details:

npm notice name:          ember-cli                               
npm notice version:       4.0.0-beta.2                            
npm notice filename:      ember-cli-4.0.0-beta.2.tgz              
npm notice package size:  259.4 kB                                
npm notice unpacked size: 1.2 MB

Problem is that !docs/build/data.json "kill" the /docs/ rule and we need to explicitly ignore files within that dir.