[Markdown] A few improvements to mentorship page

[AnthonyLatsis]

Modifications:

• Change heading from "Interest Surveys" to "Participation". The latter is a simpler and more catchy word.
• Don't repeat info that is already in the timeline.
• The program duration is measured in weeks. Express time commitment in hours/week too.
• Clarify that the exit survey is not compulsory.
• Add link to mentee interest survey

[AnthonyLatsis]

cc @devanshimodha

We posted a link to the mentee interest survey on the forums, so this change is not super urgent, but please consider prioritizing it anyway. Thanks.

[devanshimodha]

cc @devanshimodha

We posted a link to the mentee interest survey on the forums, so this change is not super urgent, but please consider prioritizing it anyway. Thanks.

Yes, thank you @AnthonyLatsis for putting up the link here as well along with the mentioned changes.

[alexandersandberg]

@swift-ci please test

[AnthonyLatsis]

Thanks, Alexander.