docs: overhaul stacking.md (remove outdated data, update branding, an…

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Description

As a Stacks documentation contributor, I would like to update the stacking.md file to reflect the current post-Nakamoto network state. This is needed because the existing documentation contains stale 2021 data, deprecated "Stacks 2.0" branding, and broken links, which are misleading for current developers.

1. Motivation for change: The stacking.md file contained outdated 2021 Stacking thresholds, used deprecated "Stacks 2.0" branding, and included a dead link to a legacy blog.
2. What was changed:
   • Removed obsolete 2021 Stacking threshold figures.
   • Replaced hardcoded values with a reference to the official Stacking documentation.
   • Updated "Stacks 2.0 blockchain" branding to "Stacks blockchain".
   • Removed the broken link to blog.blockstack.org.
3. How does this impact application developers: Improves documentation reliability and ensures developers are guided to the most current resources.
4. Link to relevant issues and documentation: N/A
5. Provide examples of use cases with code samples and applicable acceptance criteria: N/A

Type of Change

• API reference/documentation update

Does this introduce a breaking change?

No.

Are documentation updates required?

• Link to documentation updates: N/A (This PR is the documentation update)

Testing information

1. Is testing required for this change? No.
2. If it’s a bug fix, list steps to reproduce the bug: N/A
3. Briefly mention affected code paths: N/A
4. List other affected projects if possible: N/A
5. Things to watch out for when testing: Verified links redirect to expected pages.

Checklist

• Code is commented where needed
• Unit test coverage for new or modified code paths
• npm run test passes
• Changelog is updated
• Tag 1 of @person1 or @Person2