Clarify about all features BitcartCC provides

[MrNaif2018]

Currently, our documentation doesn't have all a description of possible features we provide.

We need to update it to show what is available, i.e.:

• BitcartCC Merchants API - API simplifying common actions for merchants, manages db by itself + many features like DLC support, images, discounts, templates, emails, notification sending and more. A link to https://api.bitcartcc.com swagger documentation should be added. Also, maybe a guide about how to create API token and authenticate should be added
• BitcartCC Admin Panel - Admin panel providing an easy way to manage your wallets/stores/products/invoices and more. A user interface around BitcartCC Merchants API. https://admin.bitcartcc.com https://docs.bitcartcc.com/bitcartcc-basics/walkthrough
• BitcartCC Store - a ready store template providing ability to merchants to create their stores without any programming language knowledge. It also supports 100% of Merchants API features. It is light and fast to be a responsive store.
• BitcartCC SDK - An easy to use wrapper around BitcartCC daemons, with additional utilities - it is for advanced use cases beyond merchants' use cases. https://sdk.bitcartcc.com
• BitCCL (in alpha) - a programming language for checkout flow automation - will be integrated into main deployment soon - makes it possible to avoid manual order processing, by adding a way to execute code on certain events (product bought, new invoice, user registered, etc.) of BitcartCC system.

Also some example usages should be added:

• https://github.com/MrNaif2018/bitcart-sdk/tree/master/examples examples of using SDK
• https://github.com/MrNaif2018/bitcart-site/blob/master/pages/index.vue#L79 - payment button in official site

The doc should be in integrations.md file (so it's https://docs.bitcartcc.com/integrations/ page)

Feel free to ask about certain features more for me to clarify it more