script to aid in migrating downloads pages

Figuring out the plugin system and everything so that we can generate the
download pages from structured data sounds great but a bit time-consuming;
having a script to generate all of the boilerplate is a good middle-road for
now.

The data class I ended up *not* adding here is the following (rough draft):

```python3
from dataclasses import dataclass
from typing import List

@DataClass
class Download:
    page_title: str
    description: str
    download_links: List[str]
    soar_capabilities: List[str]|None = None
    external_environment: str|None = None
    default_rules: str|None = None
    associated_publications: List[str]|None = None
    developers: List[str]
    soar_versions: List[str]
    project_type: str|None = None

ALL_DOWNLOADS = {
    "agents": [],
    "agent_dev_tools": [],
    "domains": [],
    "examples_and_unsupported": [],
}
```

See #28.

scripts/new_download_page.py

@@ -0,0 +1,128 @@
+# Generates a new download page and adds it to navigation
+# Usage: python scripts/new_download_page.py <parent> <title>
+# where <parent> is one of "agent_development_tools", "agents", "domains", "examples_and_unsupported"
+
+from pathlib import Path
+import sys
+from string import Template
+
+PROJECT_DIR = Path(__file__).parent.parent
+
+DOWNLOADS_DIR = PROJECT_DIR / "docs" / "downloads"
+
+MKDOCS_YML = PROJECT_DIR / "mkdocs.yml"
+
+SECTIONS = ["agent_development_tools", "agents", "domains", "examples_and_unsupported"]
+
+DOWNLOAD_TEMPLATE = Template(
+    """---
+Tags:
+    - TODO
+---
+
+# $title
+
+TODO: description
+
+## Environment Properties
+
+*   TODO
+
+## Download Links
+
+*   [TODO: name](TODO: link)
+
+## Associated Agents
+
+TODO (not for agents)
+
+## Documentation
+
+TODO
+
+## Associated Publications
+
+*   TODO
+
+## Developers
+
+*   TODO
+
+## Soar Versions
+
+*   TODO
+
+## Language
+
+TODO (not for agents)
+
+## Project Type
+
+TODO (agents only)
+"""
+)
+
+
+def name_to_file_name(name: str) -> str:
+    return name.lower().replace(" ", "_") + ".md"
+
+
+def main(parent: str, title: str):
+    # First check that that we are able to make all of the file changes
+    # we need to
+
+    if parent not in SECTIONS:
+        print(f"Invalid parent; must be one of {SECTIONS}")
+        return
+    parent_dir = DOWNLOADS_DIR / parent
+    index_file = parent_dir / "index.md"
+    if not index_file.exists():
+        raise FileNotFoundError(f"Parent directory {parent_dir} does not exist")
+
+    new_file_name = name_to_file_name(title)
+    new_file_path = parent_dir / new_file_name
+    if new_file_path.exists():
+        print(f"File {new_file_path} already exists")
+        return
+    new_file_contents = DOWNLOAD_TEMPLATE.substitute(title=title)
+
+    with open(MKDOCS_YML, "r") as f:
+        lines = f.readlines()
+    index_line = None
+    num_leading_spaces = -1
+    expected_parent_line = f"- downloads/{parent}/index.md"
+    for i, line in enumerate(lines):
+        if line.strip() == expected_parent_line:
+            index_line = i
+            num_leading_spaces = len(line) - len(line.lstrip())
+            break
+    line_to_insert = f"{' ' * num_leading_spaces}- {title}: downloads/{parent}/{new_file_name}\n"
+    line_inserted = False
+    for i in range(index_line + 1, len(lines)):
+        if len(lines[i]) - len(lines[i].lstrip()) <= num_leading_spaces:
+            lines.insert(i + 1, line_to_insert)
+            line_inserted = True
+            break
+    if not line_inserted:
+        lines.append(line_to_insert)
+
+    # then do the actual edits
+
+    print(f"Adding file {new_file_path}")
+    with open(new_file_path, "w") as f:
+        f.write(new_file_contents)
+
+    print(f"Adding link to {index_file}")
+    with open(index_file, "a") as f:
+        f.write(f"*   [{title}](./{new_file_name})\n")
+
+    print(f"Adding to mkdocs.yml")
+    with open(MKDOCS_YML, "w") as f:
+        f.write("".join(lines))
+
+
+if __name__ == "__main__":
+    if len(sys.argv) != 3:
+        print(f"Usage: {sys.argv[0]} <parent> <title>")
+        sys.exit(1)
+    main(sys.argv[1], sys.argv[2])