allow plugins to pass asset additions

It would be nice to allow plugins like DocumenterCitations.jl to pass css-style additions to the Documenter.jl html-writer assets.

This would allow to add own plugin specific css styles to enhance readability of elements which are added to the HTML documentation by a plugin (without starting to clutter the Documenter.jl default style).

This might also allow the creation of plugins with advanced .js capabilities (a option to limit additions of assets like .css or .js to the scope of the plugin (instead of each page) might be needed here).

Currently this could only be done by raw html insertion into the markdown (by the user or the plugin), by element wise css modification by the plugin or by manually adding a css asset .

Probably related/conflicting: #615 #982 #1433

Examples which could be improved by small plugin css additions:

DocumenterCitations.jl

```@raw html
<style>
dl {
display: grid;
grid-template-columns: max-content auto;
}
dt {
grid-column-start: 1;
}
dd {
grid-column-start: 2;
margin-bottom: 0.75em;
}
</style>

pure  |  with css addition
:-------------------------:|:-------------------------:
![bib1](https://user-images.githubusercontent.com/75576197/123248614-695aed80-d4e8-11eb-85bc-592291b7e481.png)  |   ![bib2](https://user-images.githubusercontent.com/75576197/123248618-69f38400-d4e8-11eb-8e25-e4e75170caa8.png)

[`DocStringExtensions.jl`](https://github.com/JuliaDocs/DocStringExtensions.jl)

<style>
#horlist ul {
display: grid;
grid-template-columns: repeat(auto-fit, minmax(25ch, 1fr));
grid-gap: 4px;
list-style: none;
}
</style>
<div id="horlist">

NURBStoolbox

</div>

pure  |  with css addition
:-------------------------:|:-------------------------:
![exp1](https://user-images.githubusercontent.com/75576197/123249077-e6866280-d4e8-11eb-9986-d6e55dae3295.png) | ![exp2](https://user-images.githubusercontent.com/75576197/123249078-e71ef900-d4e8-11eb-830b-64103cbd0d29.png)

[`DocStringExtensions.jl`](https://github.com/JuliaDocs/DocStringExtensions.jl)

<style>
#horfield li > p + p::before {
content: ": ";
}
#horfield li > p {
display: contents;
}
</style>
<div id="horfield">

NURBS
NURBS1D
NURBS2D

</div>


pure  |  with css addition
:-------------------------:|:-------------------------:
![field1](https://user-images.githubusercontent.com/75576197/123249259-103f8980-d4e9-11eb-8b23-43e78c82418f.png) | ![field2](https://user-images.githubusercontent.com/75576197/123249263-10d82000-d4e9-11eb-9c34-ff897ee38dbf.png)

JuliaDocs / Documenter.jl

allow plugins to pass asset additions #1612