Add a link to the nursery; tweak icons (#5637)

## Summary

We now always render the icons, but very faintly if inactive, and always
right-align. This ensures consistent alignment as you scroll down the
page:

<img width="1792" alt="Screen Shot 2023-07-09 at 10 45 50 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/da47ac0e-d646-49e1-bbe1-9f43adf94bb4">

docs/faq.md

@@ -370,6 +370,38 @@ matter how they're provided, which avoids accidental incompatibilities and simpl
 By default, no `convention` is set, and so the enabled rules are determined by the `select` setting
 alone.
 
+## What is the "nursery"?
+
+The "nursery" is a collection of newer rules that are considered experimental or unstable.
+
+If a rule is marked as part of the "nursery", it can only be enabled via direct selection. For
+example, consider a hypothetical rule, `HYP001`. If `HYP001` were included in the "nursery", it
+could be enabled by adding the following to your `pyproject.toml`:
+
+```toml
+[tool.ruff]
+extend-select = ["HYP001"]
+```
+
+However, it would _not_ be enabled by selecting the `HYP` category, like so:
+
+```toml
+[tool.ruff]
+extend-select = ["HYP"]
+```
+
+Similarly, it would _not_ be enabled via the `ALL` selector:
+
+```toml
+[tool.ruff]
+select = ["ALL"]
+```
+
+(The "nursery" terminology comes from [Clippy](https://doc.rust-lang.org/nightly/clippy/), a similar
+tool for linting Rust code.)
+
+To see which rules are currently in the "nursery", visit the [rules reference](https://beta.ruff.rs/docs/rules/).
+
 ## How can I tell what settings Ruff is using to check my code?
 
 Run `ruff check /path/to/code.py --show-settings` to view the resolved settings for a given file.