Rename model's name and and add some description

[YuChaode]

1. rename the data model and limit it to hardware scope (see #58);
2. referenced the IANA website for IANA imported module (see #58);
3. fixed revision statement (see #58);
4. add some more description to fix the issue mentioned by tom petch (see #58);
5. remove rack-number in rack, firmware-rev in NE (fix #63);
6. add relative-position of chassis in rack (fix #61);
7. remove equipment room, rack, NE's info in the component's parent reference structure.

Note that:

• ietf-network-inventory.yang has been renamed as ietf-network-hardware-inventory.yang
• ietf-network-inventory.tree has been renamed as ietf-network-hardware-inventory.tree

[italobusi]

2023-02-15 Network Inventory weekly call

The root container has to be renamed as "network-hardware-inventory" based on the clarified scope of the document

After that change, the draft PR would be ready for review

• [x] @YuChaode update the root container and start the PR review
• [x] @aguoietf @sergiobelotti @jbouqui153 review the PR once ready for review