FIX: install quantecon in networks.md

`lectures/networks.md` imports `quantecon` but its pip cell never installs
it. The lecture builds today only because `markov_chains_I`, `markov_chains_II`
and `eigen_II` appear earlier in the TOC and install `quantecon` into the same
runner environment, so a full cold build satisfies the dependency by accident.

That breaks as soon as the lecture executes on its own — the normal case with a
warm `jupyter-cache`, where only the edited notebook is re-run. Observed in the
Chinese edition, which carries the same gap as a faithful translation.

Adds `quantecon` to the lecture's own pip cell and splits the cell to one
package per line, matching `input_output.md` and the style guide.

Closes #801

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mmcky

lectures/networks.md

@@ -16,7 +16,9 @@ kernelspec:
 ```{code-cell} ipython3
 :tags: [hide-output]
 
-!pip install quantecon-book-networks==1.6 pandas-datareader
+!pip install quantecon
+!pip install quantecon-book-networks==1.6
+!pip install pandas-datareader
 ```
 
 ## Outline