revise wording for building docs tutorial

[tiffany-taylor]

This improves upon #15 which uses the directory structure from doc-base's readme. I've also revised some assumptions and notes.

I was not able to fully test the symlink section, but I believe it's correct.

[heiglandreas]

Only some minor details that tripped me up because of a dockerized setup.

my PHP "binary" looks like this:

#!/bin/bash
docker run -it --rm -v $(pwd):/repo -w /repo mileschou/xdebug:8.0 php $@

As that only mounts the current working directory that is a bit like a chroot environment that needs some tweaks for paths 😉

[tiffany-taylor]

I'm noting a comment I placed in the markdown in here, and will remove the comment from markdown. I would like to complete this as a future TODO item, but I'm thinking it will require minor refactoring of the Parsedown script.

• change text reference "Set up local php.net mirror" into a link that works with markdown and safely converts into HTML for doc.php.net. Currently [Set up a local php.net mirror](/#set-up-a-local-phpnet-mirror) does not appear to correctly convert into HTML.

[salathe]

change text reference "Set up local php.net mirror" into a link that works with markdown and safely converts into HTML for doc.php.net. Currently [Set up a local php.net mirror](/#set-up-a-local-phpnet-mirror) does not appear to correctly convert into HTML.

We should be able to get away with adding an anchor tag with an id, and then linking to that with the id. Parsedown (and really, any Markdown renderer) should be fine with that.

For example:

<a id="set-up-a-local-phpnet-mirror"></a>
## Set up a local php.net mirror

This is explained in the [Set up a local php.net mirror](#set-up-a-local-phpnet-mirror) section.

[salathe]

Thanks Tiffany!

I have added a few comments, but they're all minor. I'd be happy to see this PR merged as-is and any (or none) of my comments addressed later.