docs: fix from_source rendering and promote bold pseudo-headings

[janickm]

What

The "Deriving an ideal target (IdealPinholeCameraModelParameters.from_source)"
block in the sensor models docs rendered weirdly and its references into the API
docs were missing. While fixing that, the same bold-paragraph-as-heading pattern
was applied consistently across the docs.

Changes

from_source rendering and API links (the original report)

• The "Deriving an ideal target" and "Rectificator" blocks used bold paragraphs
  (**...**) as pseudo-headings. With a code literal inside the bold they
  rendered oddly and never showed up in the page table of contents. Promoted
  both to real RST subsection headings.
• References to IdealPinholeCameraModelParameters.from_source / natural_fov
  and Rectificator / CameraModel were plain double-backtick literals, so they
  did not link into the API docs. Converted them to :meth: / :class:
  cross-references that resolve against the autodoc'd ncore.data and
  ncore.sensors modules.

Bold pseudo-headings promoted to a real sub-hierarchy

The same **...** pseudo-heading pattern was used throughout the docs. Promoted
the remaining standalone ones to real headings with a consistent hierarchy:

• docs/data/sensor_models.rst: the camera/lidar/external-distortion model names
  (FTheta, Ideal Pinhole, OpenCV Pinhole, OpenCV Fisheye, Bivariate Windshield,
  Row-Offset Structured Spinning Lidar) become subsection headings. Each sat
  directly under a .. _label: cross-reference target, so the :ref: links that
  point at them now resolve to real titled sections (with permalink anchors) and
  appear in the page TOC. Their inline Mathematical Model / Parameters /
  Notation / Direction labels become nested sub-subsection headings.
• docs/data/formats.rst: Label Type System / Encoding /
  Instance naming convention / Compat layer access become sub-subsections of
  "Camera Labels Component".
• docs/tools/ncore_sequence_meta.rst: Global options / V4 sub-command options become subsections of "Options".
• docs/data/storage_and_access.rst: Tradeoffs becomes a subsection of the
  ".itar" format section.

Mid-paragraph emphasis (e.g. **Example:**, **Forward distortion**) is left as
bold inline text, since those are not standalone headings.

Verification

bazel build //docs:ncore succeeds with no RST or cross-reference warnings (only
pre-existing unrelated nbsphinx deprecation notices). The rendered HTML confirms:

• data/sensor_models.html: model names render as <h3> and their inline labels
  as nested <h4>; :ref: links resolve to real heading anchors (e.g.
  #ftheta-camera-model); and the from_source / natural_fov references link
  into apis/data.html#ncore.data.IdealPinholeCameraModelParameters.from_source
  (and .natural_fov).
• data/formats.html, tools/ncore_sequence_meta.html and
  data/storage_and_access.html show the promoted headings at the expected
  nesting levels.