feat: add page option and hide nav bar when generating PDF

[foreseaz]

Resolve #15 [feature] config for user to hide nav bar and change margin

[foreseaz]

Usage:

[
    "vuepress-plugin-export",
    {
        hideNav: true,
        page: {
            format: 'A4',
            printBackground: true,
            margin: {
                top: 60,
                left: 20,
                right: 20,
                bottom: 60
            }
        }
    }
]

[foreseaz]

  extendPageData($page) {
    if (opts.hideNav) {
      const {
        _context: { themeConfig }
      } = $page
      themeConfig.navbar = false
    }
  },

Directly change the _context may affect the building of original deployment. Need to find another workaroudn for hiding the navbar.

[ulivz]

Could u help resolve the conflict & update docs?