Update phone parsing

[Andrew22Teoh]

This PR does two things simultaneously.

First, it allows phone numbers to have whitespace in the middle between 2 groups of 4 numbers.
This is in line with how we had originally envisioned the way that phone numbers should be parsed, and it also makes sense from a human readability point of view. However, spaces in unexpected places will not be allowed.
Thus, 9112 3942, 91123942, and even 9112 3942 are allowed, but 911 23942 will not be allowed.
Closes #149

Secondly, it requires the first digit of the phone number to be a 6, 8, or 9. This is because we have scoped SocialBook to be for social workers in Singapore, and so any "phone number" entered by users that does not begin with those numbers must be a typo, so we want the error to notify them of it.
Closes #168

Tests have been updated accordingly, as has the user guide. For further clarification on the multiple fields we are adding, I have also created a new section in the user guide Contact field requirements, where we can specify the detailed requirements on any contact fields that aren't obvious. This might be related to the intention of #153, but I'm not sure if this is what the suggestion meant, so do let me know if this would be fine.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ	Complexity Δ
...rc/main/java/seedu/address/model/person/Phone.java	92.85% <ø> (ø)	6.00 <0.00> (ø)

[Rachael-Chan]

LGTM!