docs (max-elements-per-file): add docs

docs/rules/max-elements-per-file.md

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+# Enforces a maximum number of elements per file (max-elements-per-file)
+
+This rule disallows creating multiple `HTMLElement` classes in a single file.
+
+## Rule Details
+
+👎 Examples of **incorrect** code for this rule:
+
+```js
+// elements.ts
+class FirstElement extends HTMLElement {}
+class SecondElement extends HTMLElement {}
+```
+
+👍 Examples of **correct** code for this rule:
+
+```js
+// first-element.ts
+class FirstElement extends HTMLElement {}
+```
+
+```js
+// second-element.ts
+class SecondElement extends HTMLElement {}
+```
+
+Declaring multiple Custom Elements per file can:
+
+- Result in large files which are difficult to navigate.
+- Make a project difficult to navigate, as Custom Elements do not have a 1-1
+mapping with files.
+- Make it more difficult for bundle optimizations such as "Tree Shaking",
+"Bundle Splitting" or "Dead Code Elimination"
+
+Having one element per file eliminates these problems.
+
+## When Not To Use It
+
+If you intentionally want multiple elements in a single file then you can
+disable this rule.