[build] Add `yarn.lock` into gitignore file (#2242)
[build] Add `yarn.lock` into gitignore file (#2242)

### Motivation

We introduced docusaurus in #2206 for building the new documentation site.
docusaurus is using [yarn](https://yarnpkg.com/en/docs/install#mac-stable) for building.
It will leave `yarn.lock` file after builds.

 ### Changes

Add `yarn.lock` into gitignore file
massakam/pulsar,ArvinDevel/incubator-pulsar,massakam/pulsar,yahoo/pulsar,ArvinDevel/incubator-pulsar,merlimat/pulsar,yahoo/pulsar,massakam/pulsar,jai1/pulsar,nkurihar/pulsar,yahoo/pulsar,merlimat/pulsar,jai1/pulsar,jai1/pulsar,massakam/pulsar,nkurihar/pulsar,ArvinDevel/incubator-pulsar,nkurihar/pulsar,merlimat/pulsar,merlimat/pulsar,yahoo/pulsar,jai1/pulsar,nkurihar/pulsar,nkurihar/pulsar,ArvinDevel/incubator-pulsar,jai1/pulsar,yahoo/pulsar,nkurihar/pulsar,nkurihar/pulsar,ArvinDevel/incubator-pulsar,massakam/pulsar,jai1/pulsar,jai1/pulsar,massakam/pulsar,jai1/pulsar,ArvinDevel/incubator-pulsar,yahoo/pulsar,merlimat/pulsar,nkurihar/pulsar,jai1/pulsar,ArvinDevel/incubator-pulsar,merlimat/pulsar,nkurihar/pulsar,ArvinDevel/incubator-pulsar