Move milestone directory under Project-Description-and-Plans

As discussed in #592 this PR moves moves the milestones directory under Project-Description-and-Plans. It also removes the redundant cv32e40p directory under milestones/CV32E40P/Reports. The new directory structure is shown below:

programs
   └── Project-Description-and-Plans
           └── milestones
                    ├── README.md
                    ├── templates
                    └── CV32E40P
                             ├── README.md
                             ├── Reports
                             │       ├── 2020-11-02
                             │       ├── 2020-11-12
                             │       ├── 2020-11-17
                             │       ├── 2020-11-19
                             │       ├── 2020-11-23
                             │       ├── 2020-11-24
                             │       ├── 2020-12-15
                             │       ├── index.html
                             │       └── Waivers
                             └── RTL_Freeze_v1.0.0
                                     ├── DesignChecklist.md
                                     ├── ...other_checklists.md
                                     └── Verification_openissues.md