Improve PHP layer documentation to explain how to use LSP or ctags or gtags
**Is your feature request related to a problem? Please describe.**
Current state of the documentation:
* https://spacevim.org/use-vim-as-a-php-ide/ explains to install the gtags layer
* https://spacevim.org/layers/gtags/ recommends the installation of universal ctags

Question not answered by the documentation:
* once that's installed what to be able to use ctrl + clic working?
It seems to be willing to use "tags" file, the one we can create with `uctags -R`, but where to define the location of the tags file? `set tags+=tags,/` doesn't seem to work -> ctrl + clic still reports E433: No tags file                                                                                                                                                                             

Behavior not consistent between the documentation and the behavior:
* gtags layer documentation offers to create a tag database with `SPC m g c`, that part works well
* the search for def/reference or jump file doesn't work: `Error: global command failed. command line: global --result=ctags-x -qde 'BasePayloadAnalyzer'` (the last one is a class name)`

But if in a shell, from the folder I've opened nvim, I write that exact command reported as failed, I've results and a success code of 0:

```
$ global --result=ctags-x -qde 'BasePayloadAnalyzer'  
BasePayloadAnalyzer   10 app/Analyzers/BasePayloadAnalyzer.php abstract class BasePayloadAnalyzer {

$ echo $?
0
```

Other issues about PHP speak about LSP, and in a recent commit you added support for Intelephense or phpactor, documentation is silent about them.

**Describe the solution you'd like**
An example of how to configure a PHP project so we've tags and can use them in a PHP project.

* To be able to do ctrl + clic and something useful happens, a jump to the definition being the less surprising action.

**Describe alternatives you've considered**
**Additional context**
See below.
