Add EditorConfig section for rst files (AMReX-Codes#2273)

## Summary
Add a section for `*.rst` files for EditorConfig.

## Additional background
When editing the Sphinx `rst` source files for AmrLevel (PR AMReX-Codes#2268), I found out that the root `.editorconfig` file doesn't have a section for `rst` files. This PR adds that section, roughly based on the guidelines at https://docs.typo3.org/m/typo3/docs-how-to-document/master/en-us/GeneralConventions/CodingGuidelines.html .
Note that I haven't gauged the "disruption" from whitespace changes that might be caused by this PR.

.editorconfig

@@ -26,6 +26,23 @@ insert_final_newline = true
 trim_trailing_whitespace = false
 
 
+[*.rst]
+# Enforce UTF-8 encoding
+charset = utf-8
+
+# Unix-style newlines
+end_of_line = lf
+
+# Newline ending in files
+insert_final_newline = true
+
+# 3 space indentation
+indent_style = space
+indent_size = 3
+
+# Clean up trailing whitespace
+trim_trailing_whitespace = true
+
 [Makefile]
 # TABs are part of its syntax
 indent_style = tab