Docs: Add "Explaining Globs" documentation

docs/getting-started/6-explaining-globs.md

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+<!-- front-matter
+id: explaining-globs
+title: Explaining Globs
+hide_title: true
+sidebar_label: Explaining Globs
+-->
+
+# Explaining Globs
+
+A glob is a string of literal and/or wildcard characters used to match filepaths. Globbing is the act of locating files on a filesystem using one or more globs.
+
+The `src()` method expects a single glob string or an array of globs to determine which files your pipeline will operate on. At least one match must be found for your glob(s) otherwise `src()` will error. When an array of globs is used, they are matched in array order - especially useful for negative globs.
+
+## Segments and separators
+
+A segment is everything between separators. The separator in a glob is always the `/` character - regardless of the operating system - even in Windows where the path separator is `\\`.  In a glob, `\\` is reserved as the escape character.
+
+Here, the * is escaped, so it is treated as a literal instead of a wildcard character.
+```js
+'glob_with_uncommon_\\*_character.js'
+```
+
+Avoid using Node's `path` methods, like `path.join`, to create globs. On Windows, it produces an invalid glob because Node uses `\\` as the separator. Also avoid the `__dirname` global, `__filename` global, or `process.cwd()` for the same reasons.
+
+```js
+const invalidGlob = path.join(__dirname, 'src/*.js');
+```
+
+## Special character: * (single-star)
+
+Matches any amount - including none - of characters within a single segment. Useful for globbing files within one directory.
+
+This glob will match files like `index.js`, but not files like `scripts/index.js` or `scripts/nested/index.js`
+```js
+'*.js'
+```
+
+## Special character: ** (double-star)
+
+Matches any amount - including none - of characters across segments. Useful for globbing files in nested directories. Make sure to appropriately restrict your double-star globs, to avoid matching large directories unnecessarily.
+
+Here, the glob is appropriately restricted to the `scripts/` directory. It will match files like `scripts/index.js`, `scripts/nested/index.js`, and `scripts/nested/twice/index.js`.
+
+```js
+'scripts/**/*.js'
+```
+
+<small>In the previous example, if `scripts/` wasn't prefixed, all dependencies in `node_modules` or other directories would also be matched.</small>
+
+## Special character: ! (negative)
+
+Since globs are matched in array order, a negative glob must follow at least one non-negative glob in an array. The first finds a set of matches, then the negative glob removes a portion of those results. These are most performant when they only include literal characters.
+
+```js
+['script/**/*.js', '!scripts/vendor/']
+```
+
+If any non-negative globs follow a negative, nothing will be removed from the later set of matches.
+
+```js
+['script/**/*.js', '!scripts/vendor/', 'scripts/vendor/react.js']
+```
+
+Negative globs can be used as an alternative for restricting double-star globs.
+
+```js
+['**/*.js', '!node_modules/']
+```
+
+<small>In the previous example, if the negative glob was `!node_modules/**/*.js`, every match would have to be compared against the negative glob, which would be extremely slow.</small>
+
+## Overlapping globs
+
+Two or more globs that (un)intentionally match the same file are considered overlapping. When overlapping globs are used within a single `src()`, gulp does its best to remove the duplicates, but doesn't attempt to deduplicate across separate `src()` calls.
+
+## Advanced resources
+
+Most of what you'll need to work with globs in gulp is covered here. If you'd like to get more in depth, here are a few resources.
+
+* [Micromatch Documentation][micromatch-docs]
+* [node-glob's Glob Primer][glob-primer-docs]
+* [Begin's Globbing Documentation][begin-globbing-docs]
+* [Wikipedia's Glob Page][wikipedia-glob]
+
+[micromatch-docs]: https://github.com/micromatch/micromatch
+[glob-primer-docs]: https://github.com/isaacs/node-glob#glob-primer
+[begin-globbing-docs]: https://github.com/begin/globbing#what-is-globbing
+[wikipedia-glob]: https://en.wikipedia.org/wiki/Glob_(programming)