Adds introduction sentence to fee page. (#291)

* Adds introduction sentence to fee page.

I received some feedback that the fees page may be a bit confusing to novice users.  This addition to the top of the page attempts to help clarify things for people who may not have a good understanding of how fees on a public blockchain work.

* fix: fix header IDs

---------

Co-authored-by: Paul Razvan Berg <[email protected]>

Author: Micah Zoltu

docs/concepts/12-fees.mdx

@@ -4,6 +4,10 @@ sidebar_position: 12
 title: "Fees"
 ---
 
+During the course of using Sablier, you may encounter up to three different types of fees:
+[Interface Fees](#interface-fees), [Contract Fees](#contract-fees), and [Gas Fees](#gas-fees). Each of these fees may go
+to a different party. Below you can see a breakdown of how each fee is calculated and who receives it.
+
 ## Interface Fees
 
 The Sablier Interface charges a flat fee for certain operations. This fee is paid in the native gas token, i.e. in ETH
@@ -111,7 +115,7 @@ The fee is ~**$1** (in the gas token) per claim, regardless of the airdrop amoun
 | Ultra         | UOS       | 0 UOS            | 0                   |
 | zkSync Era    | ETH       | 0.00036 ETH      | 360000000000000     |
 
-## Gas Fee
+## Gas Fees
 
 ### Q: How are the Interface Fees charged?