Update README.md

[BellringerQuinn]

Removed the section on Environments as this is now documented in Notion and is not relevant to anyone who doesn't work at Sequence (as they won't have access to our dev env) https://www.notion.so/sequence-xyz/Unity-1d6301e08c0080839896fb6a3d9fa6a7

Also removed the ADRs from the repo as these are also documented in Notion now and aren't overly relevant to the public as they won't be actively working on the SDK's codebase https://www.notion.so/sequence-xyz/ADRs-1d7301e08c00807b8886c92af265ff07

Finally, fixed the assembly overview section to be more accurate with what we have today

Version Increment

Please ensure you have incremented the package version in the package.json as necessary.

• I have incremented the package.json according to semantic versioning
• No version increment is needed; the change does not impact SDK or Sample code/assets

Docs Checklist

Please ensure you have addressed documentation updates if needed as part of this PR:

• I have created a separate PR on the sequence docs repository for documentation updates: Link to docs PR
• No documentation update is needed for this change.