Add '_site' to the ignored folders in site.json

[Tejas2805]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [X] Enhancement to an existing feature

Fixes #587

What is the rationale for this request?

As per #587 , the default site.json generated during markbind init can be improved by ignoring _site

What changes did you make? (Give an overview)
Updated the default site.json that is generated by the markbind init command as follows:

1. Added _site to the ignore array so that these files will be ignored while copying over to the generated site's directory.

Provide some example code that this change will affect:

const globPaths = addressableGlobs.reduce((globPages, addressableGlob) =>	      
  globPages.concat(walkSync(this.rootPath, {
    directories: false,
    globs: [addressableGlob.glob],
    ignore: [CONFIG_FOLDER_NAME, SITE_FOLDER_NAME],
}).map(globPath => ({
  src: globPath,
  searchable: addressableGlob.searchable,
  layout: addressableGlob.layout,
  frontmatter: addressableGlob.frontmatter,
}))), []);	

Is there anything you'd like reviewers to focus on?

N.A.

Testing instructions:

Run the test suite / build markbind locally and test

[Tejas2805]

@marvinchin Can I get your review on this too? Thanks a lot for your time.

[marvinchin]

LGTM! Sorry for the delay, been pretty busy the last two weeks.