docs: clarify coverage architecture for readers

[AlexanderLanin]

Why

The architecture page should help repository users understand what coverage the tool measures, how test results relate to the configured coverage scope, and how the gate reaches its result. The previous page led with Bazel and coverage internals, making those concepts hard to follow for readers who maintain the configuration but do not work on the tooling itself.

What changed

• Reorganized the page around collection, scope, and evaluation, with three diagrams that show the overview, collection inputs, and gate decision.
• Defined coverage terms before using them and clarified that the scope is the code whose coverage is assessed, which can include code no selected test executes.
• Added a consistent worked example for raw and effective coverage, including the combined denominator across two libraries.
• Moved backend and Bazel implementation detail into the later sections.
• Documented current coverage-gate limitations and linked their tracking issues: Effective coverage can pass after LLVM HTML fallback drops baseline-only files #14, Effective gate treats missing coverage totals as 0% and can pass without measurements #15, Malformed justification YAML exits 1 instead of the no-verdict code 2 #16, and Effective coverage gate compares a floored percentage instead of the unrounded value #17.

[github-actions]

License Check Results

🚀 The license check job ran with the Bazel command:

bazel run //:license-check

Status: ⚠️ Needs Review

Click to expand output

[License Check Output]
Extracting Bazel installation...
Starting local Bazel server (8.6.0) and connecting to it...
INFO: Invocation ID: 68d34894-97fb-427f-aa3e-8ff75745fd28
Computing main repo mapping: 
Computing main repo mapping: 
Computing main repo mapping: 
Loading: 
Loading: 0 packages loaded
Loading: 0 packages loaded
Analyzing: target //:license-check (1 packages loaded, 0 targets configured)
Analyzing: target //:license-check (1 packages loaded, 0 targets configured)

Analyzing: target //:license-check (5 packages loaded, 6 targets configured)

Analyzing: target //:license-check (5 packages loaded, 6 targets configured)

Analyzing: target //:license-check (5 packages loaded, 6 targets configured)

Analyzing: target //:license-check (5 packages loaded, 6 targets configured)

Analyzing: target //:license-check (5 packages loaded, 6 targets configured)

Analyzing: target //:license-check (28 packages loaded, 10 targets configured)

Analyzing: target //:license-check (91 packages loaded, 10 targets configured)

Analyzing: target //:license-check (151 packages loaded, 3139 targets configured)

Analyzing: target //:license-check (151 packages loaded, 3139 targets configured)

Analyzing: target //:license-check (160 packages loaded, 3551 targets configured)

Analyzing: target //:license-check (160 packages loaded, 6245 targets configured)

Analyzing: target //:license-check (161 packages loaded, 6245 targets configured)

Analyzing: target //:license-check (174 packages loaded, 8310 targets configured)

Analyzing: target //:license-check (178 packages loaded, 11535 targets configured)

Analyzing: target //:license-check (178 packages loaded, 11535 targets configured)

INFO: Analyzed target //:license-check (179 packages loaded, 11662 targets configured).
[13 / 16] Generating Dash formatted dependency file ...; 0s disk-cache, processwrapper-sandbox ... (2 actions, 1 running)
INFO: Found 1 target...
Target //tools:license.check.license_check up-to-date:
  bazel-bin/tools/license.check.license_check
  bazel-bin/tools/license.check.license_check.jar
INFO: Elapsed time: 28.375s, Critical Path: 2.09s
INFO: 16 processes: 12 internal, 3 processwrapper-sandbox, 1 worker.
INFO: Build completed successfully, 16 total actions
INFO: Running command line: bazel-bin/tools/license.check.license_check tools/formatted.txt <args omitted>
usage: org.eclipse.dash.licenses.cli.Main [-batch <int>] [-cd <url>]
       [-confidence <int>] [-ef <url>] [-excludeSources <sources>] [-help] [-lic
       <url>] [-project <shortname>] [-repo <url>] [-review] [-summary <file>]
       [-timeout <seconds>] [-token <token>]

[Copilot]

Copilot review overview

🟢 Approval recommended

The documentation accurately reflects the implementation and linked known limitations.

Review effort: Balanced
Findings: None

What changed in this PR

Clarifies the coverage architecture from collection and scope through gate evaluation, while documenting implementation limitations.

Changes:

• Adds collection, scope, and evaluation diagrams.
• Defines raw/effective coverage with a worked example.
• Documents backend details and known gate defects.

File	Description
docs/​architecture/​index.rst	Reorganizes and expands the coverage architecture documentation.

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[github-actions]

Documentation preview for this pull request is available at:
pr-18: https://eclipse-score.github.io/coverage_tool/pr-18/

[MaximilianSoerenPollak]

Questions / Comments from @dcalavrezo-qorix have been answered.

feedback was addressed. merging as better-than-before. Can be adjusted later.