Document .rst file maximum line length of 100

In 19d2b84 ("Create book for docs", 2024-01-30) the build of the
php-src documentation has been introduced.

It is based on reStructuredText (rst) [Docutils] for its source files,
this stems from the sphinx-build utility in use to build the static HTML
pages of the php-src documentation.

The maximum line length of these text files has been set to 100
characters.

This formatting constraint is applied with the rstfmt utility [rstfmt]
via its invocation (documented in CI build instructions and README.md:)

    rstfmt -w 100 source

The `-w, --width` option takes a WIDTH argument that is "the
target line length in characters" (cf. `rstfmt --help`.)

There is also an `--ext EXT` argument option, that is "the extension of
files to look at when passed a directory" ("source" is the name of
a directory in the invocation above) and defaults to "rst".

Henceforth, the editor configuration [EditorConfig] can benefit from
documenting this expectation in the repositories .editorconfig file,
which has been introduced already earlier in 5c38fbe ("Added
editorconfig file", 2016-06-26).

[Docutils]: https://docutils.sourceforge.io/index.html "Docutils: Documentation Utilities — Written in Python, for General- and Special-Purpose Use"
[rstfmt]: https://github.com/dzhu/rstfmt "A formatter for reStructuredText"
[EditorConfig]: https://editorconfig.org/ "EditorConfig helps maintain consistent coding styles for multiple developers working on the same project across various editors and IDEs"

Author: hakre

.editorconfig

@@ -32,3 +32,7 @@ max_line_length          = 80
 
 [*.patch]
 trim_trailing_whitespace = false
+
+[*.rst]
+indent_style             = space
+max_line_length          = 100