documentation and small changes

- added docs for `Promise` and  `PromiseFactory`
- removed outdated README docs
- moved some common utilities to `PromiseUtil`
- improved efficiency of result array resizing
- added cancellation result to promise executors
- changed visibility of `PromiseJoiner` to public, and made some method names more verbose
- inlined `DeferredExecutionException` to inside `AbstractPromise`
- inlined default promise implementation to inner class in the factory
- removed necessity for base factories to provide a logger

futur-api/src/main/java/dev/tommyjs/futur/promise/CompletablePromise.java

@@ -3,10 +3,21 @@ package dev.tommyjs.futur.promise;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
+/**
+ * A {@link Promise} that can be completed.
+ */
 public interface CompletablePromise<T> extends Promise<T> {
 
+    /**
+     * Completes the promise successfully with the given result.
+     * @param result the result
+     */
     void complete(@Nullable T result);
 
+    /**
+     * Completes the promise exceptionally with the given exception.
+     * @param result the exception
+     */
     void completeExceptionally(@NotNull Throwable result);
 
 }