Docs: Add structure to existing content

[vnikolova]

Subsystem
Docs

Problem Description
Currently there's only a large Getting started topic in docs.

Solution
Split up the document into separate XML topics.

1. Get started - includes an introduction to the library and an installation guide
2. Plugins - explains the differences between the two plugins provided by the library
3. Core concepts - the main concepts and terminology explained with relation to the API
   3.1. Services
   3.2. RPC Clients
   3.3. RPC Servers
   3.4. Transport
4. Configuration - usage of RPCConfig
5. Features - send and receive flows, service fields, field declarations
6. Releases - release and versioning

Can you please check if this structure makes sense?

[Mr3zee]

Hi, @vnikolova ! Thank you for the update!
I just want to suggest a bit restructuring the topics, like this:

1. Get started
2. Plugins
3. Core concepts
   3.1. Services
   3.2. RPC Clients
   3.3. RPC Servers
4. Protocols
   4.1 kRPC
   4.1.1 Configuration
   4.1.2 Features
   4.1.3 Transport
5. Releases - release and versioning

The reason for such a structure, is that Configuration, Features (in their current way) and Transport are only applicable to kRPC protocol. Ans also there are plans for other protocols support, so we can add them to the Protocols section

Please tell me if this makes sense to you

[vnikolova]

@Mr3zee thank you for clarifying. It makes perfect sense!

[Mr3zee]

Thank you for the PR!