docs(research): expand Beacon Chain upgrades overview

[XiaoHai67890]

Summary

• Expands the Beacon Chain upgrades page from a stub into a consensus-layer upgrade overview.
• Adds context for PBS, ePBS, inclusion lists, FOCIL, preconfirmations, EIP-7251, and staking-pool related considerations.
• Adds an active-research warning and cross-links to existing local wiki pages.
• Adds bitmask and decrypt to wordlist.txt so the repository spell check passes on existing technical terms.

Checks

• Based on main
• One commit
• git diff --check upstream/main...HEAD passes
• bash check_typos.sh passes locally
• Local wiki links in the edited page resolve to existing files

[github-actions]

Hi @XiaoHai67890,

⚠️ Potential typos found in your pull request:

• 📄 ./docs/wiki/research/history-expiry/history-expiry.md (line(s) 37,39):
  1. ❌ bitmask
• 📄 ./docs/wiki/research/history-expiry/history-expiry.md (line(s) 37,39):
  1. ❌ bitmask
• 📄 ./docs/wiki/CL/cl-networking.md (line(s) 251):
  1. ❌ decrypt

ℹ️ How to fix this error:

1. If these are actual typos:

   • Open the files at the specified line numbers and fix them

2. If these are names or one-off nouns:

   • Wrap them in <name> tags
   • Example: <name>Alex Pereira</name>
   • Use this for people's names or unique terms that appear rarely

3. If these are valid terms:

   • Add them to wordlist.txt (one word per line)
   • Terms must be plain text without spaces/special chars
   • The list is case-insensitive

4. If these are code terms:

   • Wrap them in backticks (`) in your markdown

ℹ️ Checking for typos locally

1. Install aspell for your platform
2. Navigate to project root and run:

for f in **/*.md ; do echo $f ; aspell --lang=en_US --mode=markdown --home-dir=. --personal=wordlist.txt --ignore-case=true --camel-case --add-sgml-skip nospellcheck list < $f | sort | uniq -c ; done

Learn more about wordlist format

[XiaoHai67890]

Closing this PR at my request for now.