Chore: Update references from Eliza to Langchain

[OjusWiZard]

This pull request updates documentation and references to replace the Eliza agent example with the Langchain agent example. The changes ensure consistency across files and clarify which agent framework is being used in examples.

Documentation updates:

• Updated docs/olas-sdk/index.md to reference the Langchain agent example instead of the Eliza agent, including links to binary building instructions and GitHub workflow examples. Removed the Eliza agent from the list of agent blueprint examples.

Reference and spelling corrections:

• Updated HISTORY.md to note the change from Eliza to Langchain in example agent references.
• Removed "Eliza" from the .spelling file to reflect the updated example agent.