Update Community page to better reflect current processes

[aidanfnv]

Fixes #176

This change makes multiple changes to the community page on the website:

• Removes references to requiring reviews from 2 code owners
• Makes some clarifications regarding the current state of CODEOWNERS and removes the section about how to become a code owner (given that shader-slang/dev is the owner for all directories in the Slang repo)
• Fixes links to the proposals directory, which was moved from the slang repo to the spec repo, and removes mentions of its old path
• Adds a note about smaller PRs being more likely to get quick reviews (within 24 hours) than larger PRs
• Also removes some extra newlines that are ignored when deployed to the website but cause formatting issues when viewed on GitHub

Deployed site locally with Jekyll to verify that the formatting fixes do not inadvertently break the website formatting, and to confirm that the links are valid.

[jkwak-work]

Looks good to me.