linked worktrees unsupported when running `build_docs`

This is a bit of an edge case, but documenting it here for the benefit of any future folks who may run into this issue.

If `build_docs` is pointed at an ascii-doc index file in a linked [git worktree](https://git-scm.com/docs/git-worktree), it fails to identify the correct root directory for the git repo, leading to cascading failures.

**STR**
- clone Kibana
- from within the Kibana root directory, run `git worktree add ../linked-kibana-worktree`
- `cd ../linked-kibana-worktree`
- `path/to/docs/repo/build_docs --doc ./docs/index.asciidoc`

**Result**
```
INFO:build_docs:Building HTML from /doc/linked-kibana-worktree/docs/index.asciidoc
INFO:build_docs:Couldn't find repo toplevel for /doc/linked-kibana-worktree/docs
INFO:build_docs:Guessed toplevel=[/doc/linked-kibana-worktree/docs] remote=[0] branch=[master] repo=[repo]
...
```

Under the hood, the [`rev-parse` command](https://github.com/elastic/docs/blob/deed286695711d219c173d178749bd4dd953a0a8/build_docs.pl#L189) is failing with the following error (which is silenced in the current script).

```
INFO:build_docs:Error executing: git rev-parse --show-toplevel
INFO:build_docs:---out---
INFO:build_docs:
INFO:build_docs:---err---
INFO:build_docs:fatal: not a git repository: path/to/kibana/.git/worktrees/linked-kibana-worktree
```

This causes `$toplevel` to be empty and the script makes an incorrect guess and fails.