Redis 6 documentation work in progress

[antirez]

- [x] `KEEPTTL` option for `SET` command via https://github.com/antirez/redis-doc/pull/1235, merged.
- [x] `SCAN ... [TYPE type]` via https://github.com/antirez/redis-doc/pull/1137, merged.
- `INFO` via https://github.com/antirez/redis-doc/pull/1236
  - [x] modules section
  - [x] `configured_hz` server
  - [x] `tracking_clients` in clients
  - [x] `tracking_used_slots` in stats section
  - [x] `module_fork_in_progress` persistence
  - [x] `module_fork_last_cow_size` persistence
  - [x] `expired_stale_perc`
  - [x] `expired_time_cap_reached_count`
  - [x] `expire_cycle_cpu_milliseconds`
- ACL
  - [x] `AUTH` new form
  - [x] `ACL LOAD`
  - [x] `ACL SAVE`
  - [x] `ACL LIST`
  - [x] `ACL LOG`
  - [x] `ACL USERS`
  - [x] `ACL SETUSER`
  - [x] `ACL GETUSER`
  - [x] `ACL DELUSER`
  - [x] `ACL CAT`
  - [x] `ACL GENPASS`
  - [x] `ACL WHOAMI`
  - [x] `MIGRATE AUTH2`
  - [x] Add link to topics/acl.md in documentation page
  - [ ] Update topics/acl.md with the new hashed passwords format in ACL LIST, and with the new SHA256-HMAC based variable length `ACL GENPASS` command.
  - [ ] Add categories to each command page
- Client side caching
  - [x] `CLIENT GETREDIR`
  - [x] `CLIENT TRACKING`
  - [x] `CACHING`
  - [x] `OPTIN` `OPTOUT`
  - [x] Add link to topics/client-side-caching.md in documentation page
  - [x] redis.conf comments are outdated: https://github.com/antirez/redis/blob/unstable/redis.conf#L621
- RESP3
  - [ ] `HELLO` command
  - [ ] topics/protocol.md needs to be updated
- BITFIELD_RO
- [x] LCS command
- Keyspace miss notifications
- Active defrag
- TLS/SSL (https://github.com/antirez/redis-doc/issues/1181)
  - [x] https://github.com/antirez/redis-doc/pull/1253
  - Change "Encryption" to SSL/TLS in topics page
- `SPOP` return value isn't nil
- New expiry
- Modules API?
- Executable-only module libs
- https://github.com/antirez/redis-doc/issues/1216
- Add topics/gopher.md
- try.redis.io review
- topics/intro revamp to v6

Thanks to @itamarhaber for providing this table. I'm updating it as I make progress.

[itamarhaber]

Need to add CLIENT KILL by name

[mahmoudashahin]

REPLCONF is missing

[boazsegev]

It would be nice to place the RESP3 specification on the redis.io protocol documentation page.

The current situation is somewhat confusing to client library developers, especially considering that RESP2 is meant to be regarded as obsolete starting with Redis 6.0.

[madolson]

@mahmoudashahin @boazsegev Would you mind opening issues here instead? https://github.com/redis/redis-doc

Redis 6 is launched, it's probably easier to keep track of documentation over there.