[dnn] Improve error handling in dnn/linear module

[Routhleck]

This pull request includes several updates to the brainpy library, focusing on dependency management and error handling improvements. The most important changes involve adding a new dependency, refining error handling for missing packages, and updating documentation to reflect these changes.

Closes #702

Dependency Management:

• Added braintaichi to the list of dependencies in docker/requirements.txt.

Error Handling:

• Replaced PackageMissingError with raise_braintaichi_not_found() in multiple functions to provide a more specific error message when braintaichi is not found in brainpy/_src/dnn/linear.py. [1] [2] [3] [4] [5]
• Updated conditional checks to include bti in addition to ti in several functions to ensure proper handling when both dependencies are required in brainpy/_src/dnn/linear.py. [1] [2]

Documentation Updates:

• Removed references to brainpylib and updated installation instructions to reflect the new dependency requirements in docs/quickstart/installation.rst. [1] [2] [3]