Describe the main API types in the programmers guide

[kpchoi]

The "Common types" section of the programmers guide listed four types and left out the ones a reader meets first, including the instance handles oapve_t and oapvd_t.

It now groups the types a caller works with and says what each is for:

• instance handles (oapve_t, oapvd_t, oapvm_t) with their create and delete functions, what state each holds, and the note that an instance is not internally serialized, so one instance per encoding or decoding thread
• creation descriptors (oapve_cdesc_t, oapvd_cdesc_t, oapvm_cdesc_t), which configure an instance at create time
• oapve_param_t on its own, since it is the encoder setting structure rather than a descriptor, noting that it starts from oapve_param_default() and that most of it can be changed per frame at run time with oapve_config()
• application-owned buffers (oapv_imgb_t, oapv_bitb_t including the bsize versus ssize distinction, oapv_frm_t / oapv_frms_t with the pbu_type values and the role of group_id)
• types the library fills (oapve_stat_t / oapvd_stat_t, oapv_au_info_t, oapv_frm_info_t, oapv_tile_pos_t, oapvm_payload_t)

Every description was checked against the declarations in oapv.h. Documentation only; no code change.

[kpchoi]

Folded into #262, which covers the README and programmers guide updates together.