Clone some docs from our old documentation

[wpdev10]

I will add here a few docs which are there in our old documentation site and we can use them in our new doc site with no or minor modifications. Please ignore those which are already done:

• https://wpgetpaid.com/docs/translation/translate-main-plugin/
• https://wpgetpaid.com/docs/translation/getting-translation-files/
• https://wpgetpaid.com/docs/translation/handbook/
• https://wpgetpaid.com/docs/developers/wpinv_insert_invoice/
• https://wpgetpaid.com/docs/developers/wpinv_update_invoice/
• https://wpgetpaid.com/docs/addons-integrations/gravity-forms/
• https://wpgetpaid.com/docs/addons-integrations/contact-form-7-addon/
• Ninja form addon doc