Add descriptions of source compatible and binary compatible changes

[BillWagner]

Fixes #3717

Changes in this PR:

Add article that defines source breaking change and binary breaking change when using new language features in a library. This is ready for a thorough review to make sure I've expressed the concepts in the above issue correctly, and in a manner that is useful.

Add note about compatibility on some feature descriptions in what's new articles. This is the second pass of the review, if the concepts are described correctly. I took the following strategy:

1. Only add a note on features that could be part of the public API surface for a type. Other feature descriptions have not been updated. My choice is somewhat subjective. For example, I did not say anything on the feature for new syntax for numeric literals. A numeric literal could be used for a default value of an argument, but that seemed a stretch, and not useful information to add. This PR adds notes when a new feature appears in a member's signature (except for expression bodied members where there have been customer questions).
2. I tried to be concise in calling this out, but still accurate. Question for reviewers: Does that make any of the notes misleading?

The discussion on the issue referenced has examples and details that will be helpful for reviewers.

/cc @jcouv @jaredpar @JonSkeet (who wrote the comment for the original issue)

internal review link - new article
update - what's new in 6
update - what's new in 7
update - what's new in 7.2
update - what's new in 7.3
What's new in 7.1 was not updated.

[rpetrusha]

This is a really valuable topic, @BillWagner. (I've never thought about compatibility across compiler versions, only framework versions.) I've left a number of questions, comments, and suggestions.

[BillWagner]

Thanks @rpetrusha and @pkulikov

I've udpated with changes that address all the comments.

[pkulikov]

@BillWagner Thank you for addressing comments. I like a new version. Found a couple of nits to consider/fix.

[rpetrusha]

This is a really good article, @BillWagner. I've left a few more suggested changes, but you can merge when you're ready.

[jcouv]

LGTM modulo typo. Thanks!