MAHOUT-2216 [WIP] Fix Mahout Website links after pages moved to new locations

[niallkp]

Purpose of PR:

Fix incorrect links which currently point to old Unmaintained pages to proper pages in their new location.

• See https://issues.apache.org/jira/browse/MAHOUT-2216

The following pages moved from /developers to /documentation/developers

• buildingmahout.html
• developer-resources.html
• github.html
• githubPRs.html
• gsoc.html
• how-to-become-a-committer.html
• how-to-release.html
• how-to-update-the-website.html
• issue-tracker.html
• patch-check-list.html
• thirdparty-dependencies.html
• version-control.html

The following page moved from /developers to /about

• how-to-contribute.html

The following page moved from /general to /download

• downloads.html

The following pages moved from /general to /community

• mailing-lists.html
• who-we-are.html

Important ToDos

Please mark each with an "x"

• A JIRA ticket exists (if not, please create this first)[https://issues.apache.org/jira/browse/mahout/]
• Title of PR is "MAHOUT-XXXX Brief Description of Changes" where XXXX is the JIRA number.
• Created unit tests where appropriate
• Added licenses correct on newly added files
• Assigned JIRA to self
• Added documentation in scala docs/java docs, and to website
• Successfully built and ran all unit tests, verified that all tests pass locally.

Does this change break earlier versions? No

Is this the beginning of a larger project for which a feature branch should be made? No