Generate markdown snippets using weaver_forge templates

[jsuereth]

Fixes #117

Add an option where markdown snippets are generated using Jinja / weaver_forge.

• Fix an issue in cache/git resolver now that semconv repo has non-model yaml files.
• Add new hooks in weaver_forge for generating just strings from templates.
• Add optional weaver_forge support to weaver_semconv_gen for snippet generation.
• Update documentation of weaver_semconv_gen (note: maybe this one needs a better name)

Example:

weaver registry update-markdown \
  --registry=model \
  --attribute-registry-base-url=/docs/attributes-registry" \
  --templates=templates \
  --target=markdown \
  --dry_run \
  docs/

This will look for a jinja template templates/registry/markdown/snippet.md.j2 and use it to generate markdown snippets instead of hardcoded write! statements.

[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 74.8%. Comparing base (f6cecba) to head (2c34804).

Files	Patch %	Lines
crates/weaver_cache/src/lib.rs	0.0%	5 Missing ⚠️
crates/weaver_forge/src/registry.rs	89.4%	4 Missing ⚠️
crates/weaver_forge/src/error.rs	0.0%	3 Missing ⚠️
crates/weaver_forge/src/lib.rs	77.7%	2 Missing ⚠️
crates/weaver_semconv_gen/src/lib.rs	97.7%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #141     +/-   ##
=======================================
+ Coverage   71.9%   74.8%   +2.8%     
=======================================
  Files         40      40             
  Lines       2208    2288     +80     
=======================================
+ Hits        1589    1712    +123     
+ Misses       619     576     -43     

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[lquerel]

LGTM