docs: Explain upgrade process

[kostis-codefresh]

Signed-off-by: Kostis Kapelonis [email protected]

Mentioning upgrade process because people are asking about this all the time (example #2308)

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional with a list of types and scopes found here, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
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Go Published Test Results

1 798 tests   1 798 ✔️  2m 30s ⏱️
   102 suites         0 💤
       1 files           0 ❌

Results for commit 2a52b3c.

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E2E Tests Published Test Results

    2 files      2 suites   1h 31m 49s ⏱️
  89 tests   84 ✔️ 3 💤 2 ❌
182 runs  172 ✔️ 6 💤 4 ❌

For more details on these failures, see this check.

Results for commit 2a52b3c.