Update and improve docs

[tfrommen]

Please check if the PR fulfills these requirements

• [ ] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes/features)
• [ ] Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Update and improve the readme and the docs pages.

• Remove leading $ terminal indicator for shell commands.
• Add shell type for fenced code block (where missing).
• Include missing Application Flow documentation in readme.
• Replace tabs with spaces.
• Fix link to LICENSE file in docs page. (The markdown was broken, but the docs pages don't have access to files outside of the docs folder anyway.)

What is the current behavior? (You can also link to an open issue here)

What is the new behavior (if this is a feature change)?

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Other information:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.87%. Comparing base (ef0143d) to head (3cc753e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #43 +/- ## ========================================= Coverage 98.87% 98.87% Complexity 192 192 ========================================= Files 9 9 Lines 531 531 ========================================= Hits 525 525 Misses 6 6 ``` | [Flag](https://app.codecov.io/gh/inpsyde/modularity/pull/43/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=inpsyde) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/inpsyde/modularity/pull/43/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=inpsyde) | `98.87% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=inpsyde#carryforward-flags-in-the-pull-request-comment) to find out more.

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