chore: add targets for links from Ops docs

[dwilding]

This PR adds intersphinx targets needed by canonical/operator#1603. We haven't been entirely consistent with - vs _ in targets in Pebble, so I've tried to be as consistent as possible:

• Use - for title targets
• Match the format of siblings for section targets

[benhoyt]

Thanks. Currently the docs fail to build:

/home/docs/checkouts/readthedocs.org/user_builds/canonical-pebble/checkouts/581/docs/reference/index.md:33: WARNING: more than one target found for 'myst' cross-reference layer-specification: could be :std:ref:`Layers` or :std:doc:`Layers` [myst.xref_ambiguous]
/home/docs/checkouts/readthedocs.org/user_builds/canonical-pebble/checkouts/581/docs/reference/index.md:34: WARNING: more than one target found for 'myst' cross-reference layer-specification: could be :std:ref:`Layer specification` or :std:doc:`Layer specification` [myst.xref_ambiguous]

[dwilding]

Thanks. Currently the docs fail to build:

/home/docs/checkouts/readthedocs.org/user_builds/canonical-pebble/checkouts/581/docs/reference/index.md:33: WARNING: more than one target found for 'myst' cross-reference layer-specification: could be :std:ref:`Layers` or :std:doc:`Layers` [myst.xref_ambiguous]
/home/docs/checkouts/readthedocs.org/user_builds/canonical-pebble/checkouts/581/docs/reference/index.md:34: WARNING: more than one target found for 'myst' cross-reference layer-specification: could be :std:ref:`Layer specification` or :std:doc:`Layer specification` [myst.xref_ambiguous]

Hmm, I thought I had run the checks locally, but apparently not. Just pushed a fix - we should be good now.

[benhoyt]

Thanks!