Introduced background and handbook organisation, reworked text a bit

[kjellp]

Describe the pull request:

• [ ] Bug fix
• [ ] Functional change
• [ ] New feature
• [ ] Code cleanup
• [ ] Build system change
• [x] Documentation change
• [ ] Language translation

Pull request long description:

Reworked the front page according to discussions in the handbook working group.

Changes made:

1) Added brief introduction on what SDA is 2) Reworked the existing text around figure and in table to describe three main archive sub-processes: Submission, Ingestion and Data Retrieval, removed the "Status" column in table 3) Introduced the overall organisation of 4 main section in the text, and briefly their intended content. 4) Kept original "unused" text towards the bottom indicated with a note to be considered reused in section intro pages or elsewhere more suitable.

Mentions:

@norling

[kjellp]

All very good suggestions Martin, accepted and commited now:)

[blankdots]

please add the spelling words highlighted in https://github.com/neicnordic/neic-sda/actions/runs/3743236980/jobs/6355162208#step:4:1 docs/dictionary/wordlist.txt so that next time they are ignored

[kjellp]

@blankdots Thanks for the instructions, words added now :)