docs(metatag): Add metatag documentation

[joaogarin]

Added some documentation about my experience trying out metatag module. I did run into #609. I then found the GraphQL Metatag which worked well for me and seems like the easiest way to get started on it.

Even though metatag is another contrib, its probably going to be a common scenario people run into and worth documenting the use case.

Not sure if this should be targeting 8.x-3.x or the documentation branch, if that one's still alive

[codecov]

Codecov Report

Merging #619 into 8.x-3.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           8.x-3.x     #619      +/-   ##
===========================================
- Coverage    79.11%   79.09%   -0.02%     
===========================================
  Files          187      187              
  Lines         2949     2947       -2     
===========================================
- Hits          2333     2331       -2     
  Misses         616      616

Impacted Files	Coverage Δ
src/Config/GraphQLConfigOverrides.php	87.5% <0%> (-1.39%)	⬇️

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[joaogarin]

Actually while talking on the slack channel found that there is an issue with the graphql_metatag module integration . I think the PR though still makse sense, but just something to keep in mind :

query findJob($slug: String!) {
    route(path: $slug) {
      ... on EntityCanonicalUrl {
        entity {
          entityLabel
          entityMetatags {
            key
            value
          }
        }
      }
    }
  } 

returns an internal server error as an anonymous user.

[fubhy]

Cool, thanks.