Add `docc` plugin that automatically links to an EIP when it is mentioned

We reference EIPs everywhere throughout the documentation, and every time we do, we need to link to it. For example:

```python
FLOOR_CALLDATA_COST = Uint(15)
"""
Minimum gas cost per byte of calldata as per [EIP-7976]. Used to calculate
the minimum gas cost for transactions that include calldata.

[EIP-7976]: https://eips.ethereum.org/EIPS/eip-7976
"""
```

It would be fairly easy to add a new `docc` plugin, like the ones in [here](https://github.com/ethereum/execution-specs/blob/forks/amsterdam/src/ethereum_spec_tools/docc.py), that automatically finds text of the form `(EIP|ERC)-[0-9]+` and makes it a link to the relevant EIP.