[docs] Add shebang section for scripts

[sa-]

Summary

Documentation only. Adds a section in scripts.md about running uv scripts with a shebang line

Test Plan

n/a

[sa-]

CI now passes :)

[c14n]

While these changes are now formally correct, they are unfortunately factually incorrect.

The proper shebang line should be

#!/usr/bin/env -S uv run --script

The use of the -S flag for env is required due to the way spaces are handled in shebang lines. The example given in the patch will simply not work. For details, refer to

1. https://www.gnu.org/software/coreutils/env
2. «Use in shell-scripts» on https://man.freebsd.org/cgi/man.cgi?env

The use of the --script flag for uv is required to prevent infinite recursion, as seen in #11220.

Moreover, if the latter flag is used, the extension of the script can be chosen arbitrarily, or even omitted; it does not have to be .py.

[sa-]

@c14n Thanks for taking the time, I've incorporated your feedback

[c14n]

Notice that I am not a maintainer, so the following are just the observations of some random passer-by.

---

There are still some issues with this pull request, chief of which being that the changes do not mesh well with the surrounding material.

This is due to the fact that you do not adhere to the style guide, and do not try to match the structure of the surrounding material.

Notice how every section on this page of the documentation adheres to this pattern:

This is an brief introductory paragraph that explains what you can expect
to accomplish by following the advice in this section. After the colon 
terminating this sentence, we give you example code:

```python title="example.py"
# Here is the code that is absolutely necessary to illustrate the point, 
# and no more. 
```

```console
$ command-to-run-the-example-must-have-dollar-sign-before
Output. Does not start with a dollar sign.
```

Do you see how focused and concise that approach is?

My advice: Read the style guide and try to match the form of the existing material. (This also applies to commit messages.)

---

There are also some issues with the contents of the patch. I'll add inline comments to point those out.

[c14n]

An executable script with a shebang can be run from anywhere, if you provide the full path to it.

[c14n]

This is the important part of the example. Why do you distract from this with inline script dependencies?

A plain print() statement should probably be enough.

[c14n]

Does this need to be explained this explicitly? According to the style guide, guides

May assume basic knowledge of the domain.

so it should be enough to mention that a script should have its execute bit set in the introduction.

[c14n]

How about this for an introductory paragraph:

On Unix-like systems, you can turn your script into a standalone executable by setting its execute
bit with `chmod +x standalone.py` and adding a shebang interpreter directive at the start of your
script:

```python title="standalone.py"
#!/usr/bin/env -S uv run --script

print("Hello, standalone!")
```