move CPROVER manual source into git repo

[kroening]

The CPROVER manual at http://www.cprover.org/cprover-manual/ is written
using markdown. The markdown sources are translated client-side into HTML.
This PR adds the markdown sources to the repo, which enables that they are
pulled directly from the git repo.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

Approving of the overall approach. I have not actually reviewed the contents being added here, which may require some rework.

[kroening]

@tautschnig The contents will definitively need work, but that should be separate.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 633cd2e).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/89525347
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 359d30e).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/89527537