Add first unfinished draft for JSONL spec

Fix code block formatting

Update optimade.rst

Apply suggestions from code review

Apply suggestions from code review

Try to fix formatting again

Fix formatting and add note about provider fields

optimade.rst

@@ -4495,3 +4495,62 @@ Implementations that do not produce errors in this situation are RECOMMENDED to
     * XML Schema appears to use a compatible regex format, except it is implicitly anchored: i.e., the beginning-of-input ``^`` and end-of-input ``$`` anchors must be removed, and missing anchors replaced by ``.*``.
     * POSIX Extended regexes (and their extended GNU implementations) are incompatible because ``\`` is not a special character in character classes.
       POSIX Basic regexes also have further differences, e.g., the meaning of some escaped syntax characters is reversed.
+
+
+The OPTIMADE JSON Lines Format for Database Exchange
+----------------------------------------------------
+
+There are many use cases for which it is beneficial to share all of the data served by an OPTIMADE API as a single file, for example, archival, transfer of entire databases and local-first clients.
+This appendix describes a lightweight standardization for doing this via the `JSON Lines <https://jsonlines.org/>`__ format, with some additional OPTIMADE-specific conventions.
+
+The `JSON Lines <https://jsonlines.org/>`__ format enforces the following rules:
+
+- each line is a valid JSON object,
+- each line is separated by a newline character (``\n``), optionally ending the file with a newline.
+- each file must be UTF-8 encoded,
+- the recommended file extension is ``.jsonl``, with natural extensions to ``.jsonl.gz`` and ``jsonl.bz2`` for ``gzip`` and ``bzpi2`` compressed files, respectively.
+
+The OPTIMADE JSON Lines format then extends these rules with the following conventions:
+
+- The first line of the file is a JSON object that contains metadata about the file. 
+  It MUST comprise of a dictionary with the key ``x-optimade``, under which the following key MUST be defined:
+  - ``api_version``: The OPTIMADE API version used when generating the file, as described in the ``meta`` member in `JSON Response Schema: Common Fields`_.
+
+- The next line MAY contain a standard OPTIMADE ``meta`` object, following the same rules described in `JSON Response Schema: Common Fields`_, where every MUST and SHOULD rule can be reinterpreted as a MAY rule.
+- The next block of lines provides the ``info`` endpoint responses. 
+  - First the base info response MUST be provided, following the description at `Base Info Endpoint`_.
+  - The next lines MUST contain the entry info endpoint responses for the all entry types present later in the file, as described in `Entry Listing Info Endpoints`_. 
+    These MUST appear in alphabetical order by entry type name.
+- The remaining lines of the file contain data entries themselves, described in `Entry Listing JSON Response Schema`_.
+  Again, these MUST appear in block alphabetical order by entry type name, but can appear in any order within those blocks.
+- Finally, any custom extension endpoints (see `Custom Extension Endpoints`_), if present and desirable, MUST appear at the end of the file.
+
+This leaves the following overall file structure:
+
+.. code :: txt
+
+  <header>
+  <optional metadata>
+  <base info response>
+  <entry info responses>
+  <entries block ordered by entry type>
+  <optional custom extension endpoints>
+  
+
+This JSONL format can also be used to share provider-specific properties.
+These should be consistent with any external definitions, and where appropriate, prefixes tied to the tools used to generate the file should be used.
+It is RECOMMENDED that custom properties are defined in full within the JSONL file, or pointed to a specific versioned property definition.
+
+Example OPTIMADE JSON Lines File
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code :: jsonc
+
+  {"x-optimade": {"api_version": "1.2.0"}}
+  {"meta": {"time_stamp": "2024-07-19T11:47:10Z", "data_returned": 6, "provider": {"name": "Example JSONL", "description": "An example JSONL file.", "prefix": "_exmpl"}}}
+  {"type": "info", "id": "/", "attributes": {"api_version": "1.2.0", "available_api_versions": ["1.2.0"], "formats": ["json"], "entry_types_by_format": {"json": ["references", "structures"]}, "license": "https://example.com/licenses/example_license.html"}, "homepage": "https://example.com", "name": "Example API", "provider": {"description": "A simple example provider", "name": "Example Provider"}}}
+  {"type": "info", "id": "references", ...}
+  {"type": "info", "id": "structures", ...}
+  {"type": "structures", "id": "1", "attributes": {...}}
+  {"type": "references", "id": "2", "attributes": {...}}
+