Update documentation with facet tags

We can use tags to exclude certain conditions from being counted by facets.

The idea was to introduce the concept of facet exclusion. That is, to exclude facets from being altered by conditions of that same facet in the main query.

For example:

```{
searchAPISearch(index_id: "anabranch_connect_index", condition_group:{
conjunction:AND,
groups: [
{
conjunction: OR,
tags:["facet:study_fields"],
conditions: [
{operator: "=", name: "study_fields", value: "Finance"},
]
},
{
conjunction: OR,
tags:["facet:article_type"],
conditions: [
{operator: "=", name: "article_type", value: "Advice"},
]
},
]

},
```

Need to update the docs with this.