fix(docs): repair the Read the Docs build configuration

[jaelliot]

Repairs the Read the Docs build configuration, which cannot currently succeed.

Previous failure (build 30645134):

ERROR: Could not open requirements file: [Errno 2] No such file or directory: 'requirements.txt'

Root cause - four independent defects in .readthedocs.yaml:

1. python.install lists requirements: requirements.txt, but no root requirements.txt exists in the repository, so the build aborts during requirements install.
2. build.os: ubuntu-20.04 is a retired Read the Docs image.
3. The docs Python was pinned to 3.10, below the project declared requires-python of >=3.12.2.
4. Nothing installs the package, but docs/conf.py imports keria, and the module init imports hio, so the build fails on import.

What changed:

• drop the reference to the nonexistent root requirements file
• install the package itself (method: pip, path: .) so docs/conf.py can import keria
• move to ubuntu-24.04 and Python 3.14, matching the images already building successfully for the other KERI documentation projects

Validation performed on this branch:

• YAML parses and resolves to the intended values
• every referenced path exists: docs/conf.py and docs/requirements.txt
• config-only change with no application code touched; diff is .readthedocs.yaml, +5/-4
• dependency resolution checked: PyPI keri 1.3.6 declares a Python requirement of >=3.12.2 and does not carry the pin that currently affects keripy

The Read the Docs preview build is the verifying step for this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.24%. Comparing base (53a0686) to head (532592c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #463   +/-   ##
=======================================
  Coverage   88.24%   88.24%           
=======================================
  Files          26       26           
  Lines        5947     5947           
=======================================
  Hits         5248     5248           
  Misses        699      699           

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