Add .prj file to `new_known_crs` doc

The docs for `new()` and `new_known_crs()` describe several different string formats accepted for creating a new projection pipeline. These are [the options listed in `new_known_crs()`](https://docs.rs/proj/latest/proj/struct.Proj.html#method.new_known_crs):

- an "AUTHORITY:CODE", like "EPSG:25832".
- a PROJ string, like "+proj=longlat +datum=WGS84". When using that syntax, the unit is expected to be degrees.
- the name of a CRS as found in the PROJ database, e.g "WGS84", "NAD27", etc.
- more generally, any string accepted by [new()](https://docs.rs/proj/latest/proj/struct.Proj.html#method.new)

`new()` actually lists fewer options, and nothing beyond the bulleted list above.

I had a set of Shapefile files that I wanted to grab the projection from, but all I had was the `.prj` file. I didn't see that format listed in the docs, but I had nothing to lose so I tried just passing that to `new_known_crs()` and… it worked! For clarity, here's what my `.prj` looks like:

```
GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]
```

I think this is a pretty common thing that people would want to do. Just checking that this behavior is expected, and if you'd accept a PR to add this to the docs as another bullet point in `new_known_crs()`.

Start with the documentation for `new_known_crs()` and verify that the shown `.prj` string is an accepted input format. Add `.prj` or WKT-style CRS definitions as another listed format, with the existing example, and confirm the method documentation clearly describes it.

Written by the indexing model from the issue text.