fix: resolve MyST build warnings

Three warnings appear when running `myst build`:

1. basic_lessons/README.md "missing heading depth 2" — the page title is
   depth 1 but "Using this book" and "Contents" jumped straight to depth 3,
   skipping depth 2. Demote them to `##`.

2. lesson0_tutorial.md "Language is not defined for code block" — the
   verification {code-cell} had no language. Add `python` to the directive
   (``{code-cell} python``), which MyST records as the code language.

3. lesson2_tutorial.md "textEnv, Too few columns specified in the {array}
   column argument" — the 4x4 homogeneous transformation matrices H_a and
   H_a_b declared {array}{ccc} (3 columns) but have 4. Change to {cccc}.

Verified with a full `myst build --execute --html`: no warnings remain.

Co-authored-by: openhands <openhands@all-hands.dev>

Author: openhands-agent

basic_lessons/README.md

@@ -4,13 +4,13 @@ In this six-lesson tutorial, we start from the very basics of setting up your Py
 then cover scalar and matricial operations in Python using `numpy`,
 all the way until the basics of kinematic control. Until kinematic control, most is based on [@spong2020robot].
 
-# Using this book
+## Using this book
 
 Each lesson is a [MyST text notebook](https://mystmd.org/guide/notebooks-with-markdown). Each lesson can be
 opened and executed with popular IDEs, such as [VSCode](https://code.visualstudio.com) and [PyCharm](https://www.jetbrains.com/pycharm/).
 The reader is expected to follow it sequentially.
 
-# Contents
+## Contents
 
 
 | Number | Title and Link               | Content                                                                                                                                                                                                                                    |

basic_lessons/lesson0_tutorial.md

@@ -71,7 +71,7 @@ pip install numpy matplotlib
 
 You can verify that the packages are correctly installed by running the cell below.
 
-````{code-cell}
+````{code-cell} python
 import numpy as np
 import matplotlib
 import matplotlib.pyplot as plt

basic_lessons/lesson2_tutorial.md

@@ -322,7 +322,7 @@ We can also perform pose transformations using sequential right multiplications
 
 For example, consider a translation in 3D along the _World_ frame, represented by the homogeneous transformation matrix below.
 
-$$\mymatrix{H}^0_a = \mymatrix{H}_a = \left[\begin{array}{ccc}
+$$\mymatrix{H}^0_a = \mymatrix{H}_a = \left[\begin{array}{cccc}
                                          1 & 0 & 0 & 1 \\
                                          0 & 1 & 0 & 2 \\
                                          0 & 0 & 1 & 3 \\
@@ -331,7 +331,7 @@ $$\mymatrix{H}^0_a = \mymatrix{H}_a = \left[\begin{array}{ccc}
 
 Consider a rotation in 3D about the _current_ frame, represented by the homogeneous transformation matrix below.
 
-$$\mymatrix{H}^a_b = \left[\begin{array}{ccc}
+$$\mymatrix{H}^a_b = \left[\begin{array}{cccc}
                      \cos{\theta_{ab}} & -\sin{\theta_{ab}} & 0 & 0 \\
                      \sin{\theta_{ab}} & \cos{\theta_{ab}}  & 0 & 0 \\
                      0                 & 0                  & 1 & 0 \\