robots.txt

Add default robots.txt that allows bots access to all paths.
Add mix task to generate robots.txt taht allows bots access to no paths.
Document custom emojis, MRF and static_dir
static_dir documentation includes docs for the robots.txt Mix task.

docs/static_dir.md

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+# Static Directory
+
+Static frontend files are shipped in `priv/static/` and tracked by version control in this repository. If you want to overwrite or update these without the possibility of merge conflicts, you can write your custom versions to `instance/static/`.
+
+```
+config :pleroma, :instance,
+  static_dir: "instance/static/",
+```
+
+You can overwrite this value in your configuration to use a different static instance directory.
+
+## robots.txt
+
+By default, the `robots.txt` that ships in `priv/static/` is permissive. It allows well-behaved search engines to index all of your instance's URIs.
+
+If you want to generate a restrictive `robots.txt`, you can run the following mix task. The generated `robots.txt` will be written in your instance static directory.
+
+```
+mix pleroma.robots_txt disallow_all
+```