docs: add EXRAIL resource usage guide

[BanjoR]

Automatic issue closing

Fixes #1187

Addresses documentation issue #1187.

Supersedes the closed fork-only duplicate BanjoR/dcc-ex.github.io#1.

Bounded scope

• Adds docs/exrail/resource-usage.rst covering EXRAIL task recycling, flash/RAM allocation, compile-time checks, supported limits, and practical resource checks.
• Adds one resource-usage entry to the EXRAIL toctree in docs/exrail/index.rst.
• Excludes firmware/source changes, generated files, and unrelated documentation.

The content is based on the upstream EXRAIL2.h, EXRAIL2.cpp, and EXRAILAsserts.h.

Validation evidence

• PASS — clean task-owned checkout on codex/docs-exrail-compiler-resource; upstream/sphinx is the merge base; exact diff is two allowed paths, 22 additions, 0 deletions.
• PASS — git diff --check.
• PASS — focused Docutils parse of docs/exrail/resource-usage.rst with bundled Python 3.12.13.
• PASS — toctree migration check: resource-usage is present and resolves to the added page.
• PASS — no generated or unrelated files in git status.
• PASS — title underline correction in 22ce20e; the prior focused warning was Title underline too short, and the rerun passes.

Unavailable checks

• PASS - full Sphinx HTML render and whole-site build completed with the pinned requirements.txt dependencies using python -m sphinx -M html docs docs\_build\audit -W --keep-going; all 428 sources were read and HTML output was generated successfully.
• NOT RUN — no repository-specific executable test suite was discovered for this documentation-only change.
• CI — the current-head Docs workflow run is completed with action_required; GitHub reports no completed PR checks for the current head. Maintainers should rerun CI in an environment with the requirements installed.

This pull request is ready for maintainer review.

Current exact-head CI status

N/A — No BanjoR fork Actions run exists for exact head 22ce20e310663605534e08ba8679ba5be30cd625; no hosted code-test result is claimed. Local validation above is the available evidence.