feat(lib): Sphinx directive can now read files (#261)

* feat(lib): Sphinx directive can now read files

You can optionally read a file instead of the Sphinx directive's content

* fix(lib): rst syntax and docs

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

manim_slides/docs/manim_slides_directive.py

@@ -70,16 +70,29 @@ render scenes that are defined within doctests, for example::
         ...     def construct(self):
         ...         self.play(Create(dot))
 
+A third application is to render scenes from another specific file::
+
+    .. manim-slides:: file.py:FileExample
+        :hide_source:
+        :quality: high
+
+.. warning::
+
+    The code will be executed with the current working directory
+    being the same as the one containing the source file. This being said,
+    you should probably not include examples that rely on external files, since
+    relative paths risk to be broken.
+
 Options
 -------
 
 Options can be passed as follows::
 
-    .. manim-slides:: <Class name>
+    .. manim-slides:: <file>:<Class name>
         :<option name>: <value>
 
 The following configuration options are supported by the
-directive:
+directive::
 
     hide_source
         If this flag is present without argument,
@@ -110,6 +123,7 @@ import re
 import sys
 from pathlib import Path
 from timeit import timeit
+from typing import Tuple
 
 import jinja2
 from docutils import nodes
@@ -211,7 +225,17 @@ class ManimSlidesDirective(Directive):
 
         global classnamedict
 
-        clsname = self.arguments[0]
+        def split_file_cls(arg: str) -> Tuple[Path, str]:
+            if ":" in arg:
+                file, cls = arg.split(":", maxsplit=1)
+                _, file = self.state.document.settings.env.relfn2path(file)
+                return Path(file), cls
+            else:
+                return None, arg
+
+        arguments = [split_file_cls(arg) for arg in self.arguments]
+
+        clsname = arguments[0][1]
         if clsname not in classnamedict:
             classnamedict[clsname] = 1
         else:
@@ -271,20 +295,24 @@ class ManimSlidesDirective(Directive):
             "output_file": output_file,
         }
 
-        user_code = self.content
+        if file := arguments[0][0]:
+            user_code = file.absolute().read_text().splitlines()
+        else:
+            user_code = self.content
+
         if user_code[0].startswith(">>> "):  # check whether block comes from doctest
             user_code = [
                 line[4:] for line in user_code if line.startswith((">>> ", "... "))
             ]
 
         code = [
-            "from manim import *",
             *user_code,
             f"{clsname}().render()",
         ]
 
         try:
             with tempconfig(example_config):
+                print(f"Rendering {clsname}...")
                 run_time = timeit(lambda: exec("\n".join(code), globals()), number=1)
                 video_dir = config.get_dir("video_dir")
         except Exception as e:
@@ -306,9 +334,6 @@ class ManimSlidesDirective(Directive):
         RevealJS(presentation_configs=presentation_configs, controls="true").convert_to(
             destfile
         )
-        # shutil.copyfile(filesrc, destfile)
-
-        print("CLASS NAME:", clsname)
 
         rendered_template = jinja2.Template(TEMPLATE).render(
             clsname=clsname,
@@ -400,6 +425,7 @@ TEMPLATE = r"""
 
 .. raw:: html
 
+    <!-- From: https://faq.dailymotion.com/hc/en-us/articles/360022841393-How-to-preserve-the-player-aspect-ratio-on-a-responsive-page -->
 
     <div style="position:relative;padding-bottom:56.25%;">
         <iframe