Add server extensions documentation

[vinistock]

Closes #588

Add documentation for extension authors. I tried to include our motivations and goals, guidelines to avoid common pitfalls and instructions on how to build an extension.

One thing that's missing is instructions for test helpers, mainly because we didn't build those yet. Is there anything else missing that would be helpful?

[github-actions]

Benchmark results in seconds (slowest at top)

          textDocument/completion average: 0.258426 std_dev: 0.005002
          textDocument/diagnostic average: 0.038361 std_dev: 0.008532
      textDocument/selectionRange average: 0.003223 std_dev: 0.000339
   textDocument/documentHighlight average: 0.001661 std_dev: 0.000102
        textDocument/documentLink average: 0.001635 std_dev: 0.000113
            textDocument/codeLens average: 0.001628 std_dev: 0.000119
 textDocument/semanticTokens/full average: 0.001624 std_dev: 0.000272
      textDocument/documentSymbol average: 0.001621 std_dev: 5.0e-05
        textDocument/foldingRange average: 0.001443 std_dev: 9.7e-05
textDocument/semanticTokens/range average: 0.000828 std_dev: 4.0e-05
           textDocument/inlayHint average: 0.000575 std_dev: 4.5e-05
               codeAction/resolve average: 0.000556 std_dev: 6.1e-05
               textDocument/hover average: 0.000522 std_dev: 5.9e-05
    textDocument/onTypeFormatting average: 0.000101 std_dev: 5.0e-05
          textDocument/formatting average: 6.0e-05 std_dev: 0.000275
          textDocument/codeAction average: 3.3e-05 std_dev: 2.7e-05


================================================================================
Comparison with main branch:

 textDocument/semanticTokens/full unchanged
textDocument/semanticTokens/range unchanged
      textDocument/documentSymbol unchanged
        textDocument/foldingRange unchanged
          textDocument/formatting unchanged
          textDocument/diagnostic unchanged
        textDocument/documentLink unchanged
           textDocument/inlayHint unchanged
      textDocument/selectionRange unchanged
   textDocument/documentHighlight unchanged
               textDocument/hover unchanged
          textDocument/codeAction unchanged
    textDocument/onTypeFormatting unchanged
               codeAction/resolve unchanged
          textDocument/completion faster by 1.978 %
            textDocument/codeLens unchanged

[st0012]

Thanks for the detailed documentation. I think it provides enough info to help developers get started 👍
Just added a few suggestions.

[andyw8]

This is good but it's hard to review for ease of understanding since we are already heavily involved in Ruby LSP's development. It would be helpful to get some 'outsider' feedback.

[andyw8]

Should we point people to the Ruby DX Slack for help? Maybe start a #lsp-extensions channel?

[vinistock]

@andyw8 great idea. I created a channel for ruby-lsp-extensions and cited it on the docs.

[st0012]

👍