[docs] Update some links on Pulsar website (#5492)

There are some link issues on Pulsar website, check and fix them.

site2/docs/concepts-overview.md

@@ -4,7 +4,7 @@ title: Pulsar Overview
 sidebar_label: Overview
 ---
 
-Pulsar is a multi-tenant, high-performance solution for server-to-server messaging. Pulsar was originally developed by [Yahoo](http://yahoo.github.io/), it is under the stewardship of the [Apache Software Foundation](https://www.apache.org/).
+Pulsar is a multi-tenant, high-performance solution for server-to-server messaging. Pulsar was originally developed by Yahoo, it is under the stewardship of the [Apache Software Foundation](https://www.apache.org/).
 
 Key features of Pulsar are listed below:
 

site2/docs/deploy-bare-metal-multi-cluster.md

@@ -31,7 +31,7 @@ If you want to deploy a single Pulsar cluster, see [Clusters and Brokers](gettin
 > This guide shows you how to deploy Pulsar in production in a non-Kubernetes environment. If you want to run a standalone Pulsar cluster on a single machine for development purposes, see the [Setting up a local cluster](getting-started-standalone.md) guide. If you want to run Pulsar on [Kubernetes](https://kubernetes.io), see the [Pulsar on Kubernetes](deploy-kubernetes.md) guide, which includes sections on running Pulsar on Kubernetes on [Google Kubernetes Engine](deploy-kubernetes#pulsar-on-google-kubernetes-engine) and on [Amazon Web Services](deploy-kubernetes#pulsar-on-amazon-web-services).
 
 ## System requirement
-Pulsar is currently available for **MacOS** and **Linux**. In order to use Pulsar, you need to install [Java 8](http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html).
+Pulsar is currently available for **MacOS** and **Linux**. In order to use Pulsar, you need to install Java 8 from [Oracle download center](http://www.oracle.com/).
 
 ## Install Pulsar
 

site2/docs/functions-overview.md

@@ -53,7 +53,7 @@ If you implement the classic word count example using Pulsar Functions, it looks
 
 ![Pulsar Functions word count example](assets/pulsar-functions-word-count.png)
 
-To write the function in Java with [Pulsar Functions SDK for Java](functions-develop#available-apis), you can write the function as follows.
+To write the function in Java with [Pulsar Functions SDK for Java](functions-develop.md#available-apis), you can write the function as follows.
 
 ```java
 package org.example.functions;

site2/docs/getting-started-clients.md

@@ -38,7 +38,7 @@ There are also [pdoc](https://github.com/BurntSushi/pdoc)-generated API docs for
 
 For a tutorial on using the Pulsar C++ clent, see [Pulsar C++ client](client-libraries-cpp.md).
 
-There are also [Doxygen](http://www.stack.nl/~dimitri/doxygen/)-generated API docs for the C++ client [here](/api/cpp).
+There are also [Doxygen](http://www.doxygen.nl/)-generated API docs for the C++ client [here](/api/cpp).
 
 ## Feature Matrix
 Pulsar client feature matrix for different languages is listed on [Client Features Matrix](https://github.com/apache/pulsar/wiki/Client-Features-Matrix) page.

site2/docs/getting-started-standalone.md

@@ -15,7 +15,7 @@ This tutorial guides you through every step of the installation process.
 
 ### System requirements
 
-Pulsar is currently available for **MacOS** and **Linux**. To use Pulsar, you need to install [Java 8](http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html).
+Pulsar is currently available for **MacOS** and **Linux**. To use Pulsar, you need to install Java 8 from [Oracle download center](http://www.oracle.com/).
 
 > #### Tip
 > By default, Pulsar allocates 2G JVM heap memory to start. It can be changed in `conf/pulsar_env.sh` file under `PULSAR_MEM`. This is extra options passed into JVM.