docs(readme): Update all outdated links

[seii]

Purpose of this PR

fixes #3688

All links in the README (save for two) were outdated or broken and required updating. Most notably, the link to Unity's Discord was invalid due to pointing at a deprecated Discord server.

Changelog

• Fixed: Broken Discord links
• Changed: All shield icon links (except "release" link)
  • NOTE: "docs | api" link did not have a "latest" option, so it was pointed at v2.6 (the current version)
• Changed: Links to docs and/or package sites
• Changed: Link to "Hello World" example
  • NOTE: "Hello World" link did not have a "latest" option, so it was pointed at v2.6 (the current version)
• Changed: Link to Multiplayer roadmap

Jira ticket

• None

Documentation

• No documentation changes or additions were necessary outside the README.

Testing & QA (How your changes can be verified during release Playtest)

• All links manually tested in local web browser

Backports

• None

[unity-cla-assistant]

All committers have signed the CLA.

[EmandM]

This all looks good. For the next steps I will pull these changes onto an internal branch and land this PR from there. That way we can run our tooling against this change.

Thank you for this contribution!