docs: Use 'req@url' syntax to install from remote VCS

[matthewfeickert]

Description

Resolves #2114

• Use req@url syntax when using pip to install from a remote Git repository over url#egg=req to avoid the use of #egg= fragments with a non-PEP 508 name. This will be required in pip v25.0+. c.f. Restrict #egg= fragments to valid PEP 508 names pypa/pip#11617 for more details. This avoids pip throwing the following warning:

DEPRECATION: git+https://github.com/scikit-hep/pyhf.git#egg=pyhf[all] contains an egg fragment with a non-PEP 508 name pip 25.0 will enforce this behaviour change. A possible replacement is to use the req @ url syntax, and remove the egg fragment. Discussion can be found at pypa/pip#11617

• For best practice, quote the use of extras for compatibility across all shells.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Use 'req@url' syntax when using pip to install from a remote git
  repository over 'url#egg=req' to avoid the use of #egg= fragments
  with a non-PEP 508 name. This will be required in pip v25.0+.
   - c.f. https://github.com/pypa/pip/pull/11617 for more details.
* For best practice, quote the use of extras for compatibility across all shells.

[codecov]

Codecov Report

Base: 98.30% // Head: 98.30% // No change to project coverage 👍

Coverage data is based on head (5e22958) compared to base (e80359f).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2115   +/-   ##
=======================================
  Coverage   98.30%   98.30%           
=======================================
  Files          69       69           
  Lines        4531     4531           
  Branches      645      645           
=======================================
  Hits         4454     4454           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	97.88% <ø> (ø)
doctest	61.15% <ø> (ø)
unittests-3.10	96.31% <ø> (ø)
unittests-3.8	96.33% <ø> (ø)
unittests-3.9	96.35% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyhf/cli/complete.py	100.00% <ø> (ø)
src/pyhf/cli/rootio.py	92.59% <ø> (ø)
src/pyhf/contrib/cli.py	100.00% <ø> (ø)
src/pyhf/contrib/utils.py	100.00% <ø> (ø)

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[matthewfeickert]

Relevant RTD build: https://pyhf--2115.org.readthedocs.build/en/2115/installation.html

[matthewfeickert]

I'm going to approve and merge this myself to keep things moving. As always, PRs approved by a single core dev can be reverted as needed by the rest of the dev team.