fix: delete the instance configuration in make clean

[erral]

We already have make remove-data to remove the data, make clean should not wipe the data. See #91 and #103

Fixes #213

[stevepiercy]

I ran through the project generation, cd project-title, make install. Here's the resulting directory tree.

$ tree -L 5 backend/instance/  
backend/instance/
├── etc
│   ├── site.zcml
│   ├── zope.conf
│   └── zope.ini
└── var
    ├── blobs
    │   ├── 0x00
    │   │   └── 0x00
    │   │       └── 0x00
    │   └── tmp
    ├── cache
    │   ├── 45a89972949c31ca1bed45280d2a45c3.py
    │   ├── __pycache__
    │   │   ├── 45a89972949c31ca1bed45280d2a45c3.cpython-312.pyc
    │   │   ├── a9262aacc5bd3bff9f2562d46ba123af.cpython-312.pyc
    │   │   ├── b32eaa48df8a412b3f7d07e9671e315d.cpython-312.pyc
    │   │   └── cf0de291133736baf931adb6c10f0633.cpython-312.pyc
    │   ├── a9262aacc5bd3bff9f2562d46ba123af.py
    │   ├── b32eaa48df8a412b3f7d07e9671e315d.py
    │   └── cf0de291133736baf931adb6c10f0633.py
    ├── filestorage
    │   ├── Data.fs
    │   ├── Data.fs.index
    │   ├── Data.fs.lock
    │   └── Data.fs.tmp
    └── log

12 directories, 15 files

Also there are two open PRs that relate to this issue:

• Reference Guide for Cookieplone make commands documentation#1934
• Migrate backing up Plone docs documentation#1919

I think it's OK to merge this PR, assuming that the open documentation PRs are both aligned with reality.

[ksuess]

I like your improvement!

[davisagli]

LGTM