Refactor policy package doc into separate docs for clarity

Author: rdimaio

docs/operator/policy_packages.md

@@ -0,0 +1,4 @@
+{
+    "label": "Policy Packages", 
+    "collapsible": true
+}

docs/operator_policy_packages/_category_.json

@@ -0,0 +1,45 @@
+---
+id: policy-package-algorithms
+title: Developing Policy Package algorithms
+---
+
+### Adding a new algorithm class
+
+The system for registering algorithms within policy packages is
+intended to be extensible so that new algorithm classes can be added
+relatively easily. The basic workflow is as follows:
+
+- The `get_algorithms` function within the policy package (see above)
+  should return a dictionary of functions of the new class, indexed
+  by name
+- The core Rucio code should maintain a dictionary of functions of the
+  new class, ready to be called when required. The details of this
+  will differ depending on what the new class actually does and how it
+  integrates with the Rucio code, but typically the algorithm name to
+  be used will be selected by a value in the config file, as for the
+  current `lfn2pfn` and `non_deterministic_pfn` algorithm types.
+
+### lfn2pfn vs. non_deterministic_pfn algorithms
+
+`lfn2pfn` algorithms and `non_deterministic_pfn` algorithms are
+conceptually similar, but there are important differences between
+them. Both produce a physical filename for use on an RSE, however
+`lfn2pfn` algorithms can only be used on deterministic RSEs - for
+example, disk systems where the appropriate physical filename can be
+derived from the file's scope and name alone (as well as
+protocol-specific information for the RSE in question).
+`non_deterministic_pfn` algorithms are used on non-deterministic
+RSEs (most often tape systems), and may use additional information
+about the file (such as its metadata, any datasets that it is a part
+of, etc.) to construct the physical filename. Because files cannot
+be uploaded directly to non-deterministic storage,
+`non_deterministic_pfn` algorithms are only ever called for
+replications, but `lfn2pfn` algorithms can also be called for
+initial uploads.
+
+The `lfn2pfn` algorithm to be used is determined by the
+`lfn2pfn_algorithm` attribute of the relevant RSE. If this is not set,
+the `lfn2pfn_algorithm_default` value from the `[policy]` section of
+the config file is used instead. The `non_deterministic_pfn` algorithm
+to be used is determined by the `naming_convention` attribute of the
+relevant RSE.