Add a more low-level LanguageServer.send example to the documentation

If I understand it correctly, SyncLanguageServer currently supports only a few API methods directly: request_definition, request_references, request_completions, request_document_symbols, request_hover.

For everyhing else user should use SyncLanguageServer.language_server.server.send.
Would be great to attach some example to the documentation on how to do so. Perhaps in a form of test and just refer to it.

I'll attach a simple example below.

import asyncio
from multilspy import SyncLanguageServer
from multilspy.multilspy_config import MultilspyConfig
from multilspy.multilspy_logger import MultilspyLogger
from multilspy.lsp_protocol_handler.lsp_constants import LSPConstants
from pathlib import Path, PurePath


config = MultilspyConfig.from_dict({"code_language": "python"})
logger = MultilspyLogger()
repository_root_path = r"L:\pomoika\test_files"
lsp = SyncLanguageServer.create(config, logger, repository_root_path)

# Start the language server
with lsp.start_server():
    file_path = "test.py"
    line_number, column_number = 1, 0
    server = lsp.language_server

    async def req_definition():
        with server.open_file(file_path):
            response = await server.server.send.declaration(
                {
                    LSPConstants.TEXT_DOCUMENT: {
                        LSPConstants.URI: Path(str(PurePath(server.repository_root_path, file_path))).as_uri()
                    },
                    LSPConstants.POSITION: {
                        LSPConstants.LINE: line_number,
                        LSPConstants.CHARACTER: column_number,
                    },
                }
            )
            return response

    async def req_declaration():
        with server.open_file(file_path):
            response = await server.server.send.declaration(
                {
                    LSPConstants.TEXT_DOCUMENT: {
                        LSPConstants.URI: Path(str(PurePath(server.repository_root_path, file_path))).as_uri()
                    },
                    LSPConstants.POSITION: {
                        LSPConstants.LINE: line_number,
                        LSPConstants.CHARACTER: column_number,
                    },
                }
            )
            return response

    result = asyncio.run_coroutine_threadsafe(req_definition(), lsp.loop).result(timeout=lsp.timeout)
    # [{'uri': 'file:///l:/pomoika/test_files/test.py', 'range': {'start': {'line': 1, 'character': 0}, 'end': {'line': 1, 'character': 1}}}]
    print(result)
    result = asyncio.run_coroutine_threadsafe(req_declaration(), lsp.loop).result(timeout=lsp.timeout)
    # [{'uri': 'file:///l:/pomoika/test_files/test.py', 'range': {'start': {'line': 1, 'character': 0}, 'end': {'line': 1, 'character': 1}}}]
    print(result)

Review the existing documentation for SyncLanguageServer and the low-level language_server.server.send entry point. Use the supplied declaration example as the reference, add a documented example for calling an otherwise unsupported LSP method, and verify that the example matches the public API.