Mono repo template

[YooSunYoung]

It'll be easier to add repos in the future with this template.
It renders repeating stuff in the readme and pixi.toml.

I removed the issue template because it can be directly maintained by the ess project.

I tried copier locally with the template. It doesn't have any conflict with the existing subpackages.

~/Projects/ess$ rm .copier-answers.ess.yml && git restore pixi.toml && copier copy ../ess_template/ ./
/home/sunyoungyoo/.pixi/envs/nopy/lib/python3.14/site-packages/copier/_vcs.py:209: DirtyLocalWarning: Dirty template changes included automatically.
  warn(
No git tags found in template; using HEAD as ref
🎤 Subpackages information. Name, description and dependencies.
    (Finish with 'Alt+Enter' or 'Esc then Enter')
> - dependencies: []
    description: Common data reduction tools (core)
    name: essreduce
  - dependencies:
    - essreduce
    description: Neutron imaging (ODIN, TBL, YMIR)
    name: essimaging

Copying from template version 0.0.0.post20.dev0+2aa88da
 identical  README.md
 identical  pixi.toml
    create  .copier-answers.ess.yml

@MridulS will you have a look?