[Doc] Add Arrow Example (#3236)

* [Doc] Add Arrow Vector Example

* [Doc] Add Arrow Vector Example

* [Doc] Move arrow vector example to line charts

Co-authored-by: Stefan Binder <binder_stefan@outlook.com>

* Update language

Co-authored-by: Stefan Binder <binder_stefan@outlook.com>

* replace with double quotes

* Fix language as argument syntax example

* showing 2 arrow methods in single chart

* Delete tests/examples_methods_syntax/line_chart_with_arrows.py

* Simplify code a bit

---------

Co-authored-by: Stefan Binder <binder_stefan@outlook.com>
Co-authored-by: Joel Ostblom <joelostblom@users.noreply.github.com>

tests/examples_arguments_syntax/line_chart_with_arrows.py

@@ -0,0 +1,65 @@
+"""
+Line Chart with Arrows
+----------------------
+This example shows a simple line chart with two types of arrow annotations.
+The Unicode character approach is simpler, 
+while the line plus triangle :ref:`point mark <user-guide-point-marks>` allows for greater flexibility,
+such as customizable arrowheads.
+"""
+# category: line charts
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+x = np.linspace(1,7)
+data = pd.DataFrame({
+    "x": x,
+    "y": np.sin(x),
+})
+
+unicode_arrow_with_text = alt.layer(
+    # Arrow
+    alt.Chart().mark_text(size=60, align="left", baseline="bottom", fontWeight=100, angle=340).encode(
+        x=alt.datum(2.8),
+        y=alt.datum(-0.3),
+        text=alt.datum("🠃")  # Any unicode symbol could be used instead
+    ),
+    # Text
+    alt.Chart().mark_text(size=14, align="center", baseline="bottom").encode(
+        x=alt.datum(1.8),
+        y=alt.datum(-0.1),
+        text=alt.datum("decreasing")
+    )
+)
+
+mark_arrow_with_text = alt.layer(
+    # Arrow line
+    alt.Chart().mark_line(size=2).encode(
+        x=alt.datum(5.4),
+        y=alt.datum(-0.4),
+        x2=alt.datum(5.9),
+        y2=alt.datum(0)
+    ),
+    # Arrow head
+    alt.Chart().mark_point(shape="triangle", filled=True, fillOpacity=1).encode(
+        x=alt.datum(5.9),
+        y=alt.datum(0),
+        angle=alt.AngleValue(23),
+        size=alt.SizeValue(100),
+        color=alt.ColorValue("#000000")
+    ),
+    # Text
+    alt.Chart().mark_text(size=14, align="center", baseline="bottom").encode(
+        x=alt.datum(4.7),
+        y=alt.datum(-0.3),
+        text=alt.datum("increasing")
+    ),
+)
+
+line_with_points = alt.Chart(data).mark_line(point=True).encode(
+    x=alt.X("x"),
+    y=alt.Y("y"),
+)
+
+line_with_points + unicode_arrow_with_text + mark_arrow_with_text