* markdown lint fixes for 8.0 proposals
Summary:
1. a few header formatting fixes
1. use `csharp` or `antlr` as the language identifier for code fences.
1. update links to relative links for articles published on docs.
* change the links to records.md to absolute links
When published on docs.microsoft.com, the links from these proposals will resolve to the source on dotnet/csharplang.
That's the desired behavior for records.md, because that is a proposal feature that has not been implemented.
