alt tag content missing from contributor template

- `all-contributors-cli` version: v6.20.0
- `node` version: v12.22.12
- `npm` (or `yarn`) version: 8.3.0

Relevant code or config

[format-contributor.js](https://github.com/all-contributors/cli/blob/master/src/generate/format-contributor.js#L5)

```javascript
''
```

Problem description:

The image for a contributor has an empty alt text attribute, which looks like it was added in #138 and removed in #218. While best practices for alt-text _is_ to not include it for decorative images, which was the justification for removing it, I contend that profile pictures are not decoration. Profile pictures are part of the contributor's identity -- if they weren't, then there would be no reason for including them in the first place. In my opinion, they fall under the [informative images classification](https://www.w3.org/WAI/tutorials/images/informative/) under the WC3's WAI. More detail can be found in the [WCAG standard itself](https://www.w3.org/TR/WCAG20/#text-equiv).

I'd also reference the [decision tree](https://www.w3.org/WAI/tutorials/images/decision-tree/), which doesn't lend itself to the conclusion that the profile picture is decorative.

Suggested solution:

That being said, the original implementation didn't add much value beyond the name of the profile user. We have the information, so a more comprehensive alt text string can be composed.

I'm currently using `alt=\"<%= contributor.name %>'s GitHub profile picture\"/>` in my template, but other options exist, such as:

* Profile picture of {name}, whose bio is {bio}
* Profile picture of {name}, who has {contribution stats} (using the [contextual user API](https://docs.github.com/en/rest/users/users#get-contextual-information-for-a-user))

or pick your preference.

There is a case to be made that since these images are actually links, then they should adhere to the [functional images classification](https://www.w3.org/WAI/tutorials/images/functional/), in which case the alt text should indicate where it is linking. That might look like "Profile picture of contributor linking to {link}", but it also might be better then to only link to their `html_url` and then be able to standardize on "Picture of contributor linking to their GitHub profile".

Thoughts?