Update contribute.html to include content requirements etc

[khanspers]

Currently we don't specify anywhere on the website what the requirements are for pathway models to be included on the website (aka Approved). We only have the requirements listed in various Academy stages. An idea came up to use the contribute.html page to better explain this, and in general to make the page more in the style of webpages that journals/publishers would have called "For authors". So in this case "For contributors".

When the page is ready we can link to it from the top navigation bar on the front page.

https://www.wikipathways.org/contribute.html

[khanspers]

Updated the "contribute" page to include more specific information, oragnized in three categories:

• requirements for pathway content
• Getting started
• Review process

Considering adding a table of contents like the main Help page. Any feedback is welcome.

https://www.wikipathways.org/contribute.html

[khanspers]

Updated text a bit and and added horizontal rule breaks.