Roadmap for 7.3 data frame documentation

[lcawl]

• Update conceptual overview to mention continuous checkpointing: [DOCS] Updates conceptual overview with checkpointing and continuous data frames #412
• Update data frame example likewise: [DOCS] Updates screenshots in data frame example #410
• Add sync property to create data frame transform API: [DOCS] Adds sync to data frame transform API elasticsearch#44254
• Add lower-level details about how data frames work. Part one: Adds checkpoint overview lcawl/stack-docs#3
• Blurb for 7.3 Kibana and Elasticsearch release highlights: [DOCS] Updates to data frame transforms release highlight elasticsearch#44907
• Limitations See [DOCS] Updates data frame limitations for 7.3. #407
• Troubleshooting: Adds data frame transform troubleshooting info #431
  - [ ] Add get data frame transform statistics resource details See [DOCS] Defines data frame transform stats API objects elasticsearch#44197
• Add source and dest objects inline in API (rather than as separate pages) [DOCS] Clarifies data transform source definition elasticsearch#44504 and [DOCS] Improves description of dest data frame transform object elasticsearch#44509
• Rename Data frame APIs and stack docs Data frames to Data frame transforms : [DOCS] Renames data frame APIs section elasticsearch#44458
• Describe scenarios where it makes sense to use data frames instead of aggregations: Adds info about when to use data frames #435