Update project documentation structure and clarity

[dagood]

A few people tried to contact us recently and didn't all find the channels they were expecting. (If anyone sees this: issues are great! Pinging individual developers internally is also fine, but causes delayed responses.) With that in mind, took another look at these docs.

The main change is to focus the "Support" section on product support and move "what versions of Go do you support" to its own distinct section. I expect that some people see how the Support section starts and ignore the rest because it's the wrong kind of support.

Including a hint about what support details people will find on the internal page may entice users to go there. Sometimes eng.ms doesn't load cleanly (for me) and without knowing what I'm going to find there, I might not browse it. (This might also be a useful hint for anyone asking Copilot to find these details.)

Leave SUPPORT.md to be specific to GitHub, but include a link back to the readme to make the internal support channels more discoverable.

Add a "(tar.gz and zip)" note to the "Binary archive" link. That list has gotten busier, and sometimes I don't remember that name for what I'm looking for, and these terms stand out more. (Not all that related to the other changes, just a personal nitpick.)

Improve eng/doc/README.md: simplify the links by using descriptive names from headers, fill out the list, and tidy up.

[Copilot]

Pull request overview

Updates repository documentation to make support/contact channels easier to discover and to separate product support guidance from the Go release-support policy, improving readability for both internal and external readers.

Changes:

• Refocuses README.md “Support” section on support channels and moves Go version support details into a distinct “Release cycle and policy” section.
• Adds a cross-link in SUPPORT.md pointing Microsoft-internal readers back to the README’s support resources.
• Restructures eng/doc/README.md with clearer headings and a more complete, descriptive index of eng/doc documentation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
SUPPORT.md	Adds a TIP callout linking to README support resources (Microsoft-internal channels discoverability).
README.md	Reorganizes support vs release policy content; clarifies release announcement channels; adds “tar.gz and zip” note for binary archive link.
eng/doc/README.md	Rewrites the doc index with clearer structure, maintained-branch notice, and more descriptive link names.