Clarify intro in README and manual

[Wilfred]

The first sentence a new user should see should ideally answer the questions:

• What is rust-analyzer?
• Why might I want to use it?

The vast majority of users will be interested in using rust-analyzer inside their favourite editor. We should clarify that rust-analyzer is an LSP implementation and that it supports all the classic IDE features.

Whilst it's also true that rust-analyzer is modular and organised into libraries, the first impression should (I think) focus on an overview and the primary use case.

[Wilfred]

For comparison, here's how some other LSPs introduce themselves:

Rust Language Server (RLS)

The RLS provides a server that runs in the background, providing IDEs, editors, and other tools with information about Rust programs. It supports functionality such as 'goto definition', symbol search, reformatting, and code completion, and enables renaming and refactorings.

https://github.com/rust-lang/rls

What is clangd?

clangd understands your C++ code and adds smart features to your editor: code completion, compile errors, go-to-definition and more.

clangd is a language server that can work with many editors via a plugin. Here’s Visual Studio Code with the clangd plugin, demonstrating code completion:

https://clangd.llvm.org/

gopls, the language server for Go

gopls (pronounced "Go please") is the official language server for Go, developed and maintained by the Go team. It provides a wide variety of IDE features to any LSP-compatible editor.

https://pkg.go.dev/golang.org/x/tools/gopls

[ChayimFriedman2]

While I agree that being an LSP implementation is more important to users of r-a, I think adding a sentence about it being a library for Rust analysis is also worthwhile.

[Wilfred]

How about this? I've linked the existing architecture docs too.

[ChayimFriedman2]

That looks good, thanks!