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- removed outdated README docs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/function/ExceptionalConsumer.java

@@ -1,8 +1,20 @@
 package dev.tommyjs.futur.function;
 
+/**
+ * Represents an operation that accepts a single input argument and returns no result,
+ * and may throw an exception. This is a functional interface whose functional method is {@link #accept(Object)}.
+ *
+ * @param <T> the type of the input to the operation
+ */
 @FunctionalInterface
 public interface ExceptionalConsumer<T> {
 
+    /**
+     * Performs this operation on the given argument, potentially throwing an exception.
+     *
+     * @param value the input argument
+     * @throws Exception if unable to compute a result
+     */
     void accept(T value) throws Exception;
 
-}
+}